(UiUumbia  llnmerattg 
in  tin?  (Ettg  0f  N^tu  fnrk 


^A^Uunn  Sibrarg 


Normal  Histology 

WITH  SPECIAL  REFERENCE 
TO  THE 

STRUCTURE  OF  THE  HUMAN   BODY 


BY 

GEORGE  A.  PIERSOL,  M.D.,  Sc.D. 

PROFESSOR  OF  ANATOMY  IN  THE  UNIVERSITY  OF  PENNSYLVANIA 


438  ILLUSTRATIONS,    MANY   OF   WHICH   ARE   IN   COLORS 


EIGHTH  EDITION 
{Re-'ifritteri) 


PHILADELPHIA  &  LONDON 

J.  B.  LIPPINCOTT  COMPANY 


Copyright,  1910 
By  J.  B.  LiPPiNCOTT  Company 


Printed  by  J.  B.  Lippincoti  Company 
The  Washington  Square  Press,  Philadelphia,  U.S.A. 


PREFACE. 

The  preparation  of  the  present  edition,  undertaken  in  response  to  re- 
peated requests  from  teachers,  has  been  influenced  by  the  aim  stated  in  the 
(original  preface  :  to  present  descriptions  which  should  include  the  salient 
features  of  the  various  structures  with  sulificient  fulness  to  impress  important 
details  without  wearying  minutiae  ;  too  great  conciseness,  on  the  one  hand, 
and  too  great  elaboration  of  detail,  on  the  other,  are  alike  unsatisfactory. 

The  fact  that  histology  has  its  place  at  the  beginning  of  the  medical 
curriculum,  and,  indeed,  with  increasing  frequency  in  the  courses  preparatory 
to  medicine,  often  requires  the  student  to  study  the  microscopic  details  of 
organs  before  he  has  become  acquainted  with  their  gross  anatomy.  In  order 
to  minimize  this  deficiency,  the  histological  descriptions  have  been  prefaced 
with  references  to  the  macroscopic  features  whenever  such  explanations 
seemed  desirable.  The  criticism,  for  instance,  that  an  outline  description  of 
the  brain,  by  pencil  as  well  as  by  pen,  is  out  of  place  in  a  text-book  of  his- 
tology, loses  much  of  its  force  when  the  usual  ignorance  of  the  student  con- 
cerning even  the  chief  subdivisions  of  the  central  nervous  system  is  recalled. 
The  author  believes,  therefore,  that  these  additions  are  justified  and  will  ma- 
terially facilitate  the  student's  appreciation  of  the  correlation  between  the 
structures  demonstrated  in  the  dissecting  room  and  the  details  seen  under 
the  microscope. 

With  the  exception  of  those  from  other  sources,  duly  acknowledged  in 
the  legends,  the  illustrations  have  been  drawn,  with  the  aid  of  the  camera 
lucida,  by  Mr.  Louis  Schmidt,  whose  skilful  pencil  has  faithfully  represented 
the  preparations.  The  latter,  mostly  from  the  author's  cabinet,  have  been 
selected  to  show  the  usual  rather  than  the  unusual  histological  appearances. 

The  author  gratefully  acknowledges  the  kindness  of  Prof.  William  G. 
Spiller,  Prof.  G.  Carl  Huber,  Prof.  Thomas  G.  Lee,  and  Dr.  Joseph  P.  Tunis 
in  placing  their  preparations  at  his  disposal.  To  Dr.  W.  H.  F.  Addison 
his  thanks  are  due  for  repeated  courtesies  in  preparing  specimens  and  for 
suggestions  regarding  microscopical  technique.  The  author  wishes  to  ex- 
press his  obligations  to  the  publishers  for  their  hearty  cooperation. 

University  ov  Pennsylvania, 

August,  ig/o. 


CONTENTS. 


THE   CELL. 

PAGE 

General  Considerations i 

Structure  of  the  Cell 2 

The  Cytoplasm 2 

The  Nucleus  3 

The  Nucleolus  4 

The  Centrosome  4 

Vital  Phenomena 5 

Metabolism 5 

Growth 5 

Reproduction 5  1 

Irritability 6  ; 

Cell-Division 6  ' 

Mitotic  Division 7  I 

Synopsis  of  Mitosis  10  ! 

Amitotic  Division 10 

Origin  and  Differentiation  of  Cells  .  .  11 

The  Germ-Cells 11   : 

The  Ovum 11 

The  Spermatozoon  12  , 

Segmentation 12  : 

Blastodermic  Vesicle  13  j 

The  Germ-Layers 13  , 

Derivatives  of  the  Germ-Layers .  15! 

THE     ELEMENTARY    TISSUES. 

The  Epithelial  Tissues 16 

Squamous  Epithelium 16 

Columnar  Epithelium 18 

Modified  Epithelium 18 

Endothelium 20 

The  Connective  Tissues 21 

Mucous  Tissue  22 

Reticular  Tissue 22 

Fibrous  Tissue 23 

Tendon 28 

Adipose  Tissue 29 

Hyaline  Cartilage   31 

Elastic  Cartilage 33 

Fibrous  Cartilage 33 

Bone    34 

Periosteum 38 

Bone-Marrow 39 

Red  Marrow 40 

Development  of  Bone 42 

Endochondral  Formation  . .  43 

Intracartilaginous 43 

Subperiosteal 47 

Intramembranous  Formation  49 


PACK 

Development  of  Bone — Continued. 

Growth  of  Bones  50 

Articulations 51 

Synarthrosis   51 

Diarthrosis 51 

Muscular  Tissue 53 

Nonstriated  Muscle 53 

Cardiac  INIuscle 55 

Striated  Muscle 57 

Development  of  Muscle  61 

Attachment  of  Muscles 62 

Aponeuroses 63 

Fasciae 63 

Tendon-Sheaths 63 

Bursae 64 

The  Nervous  Tissues 64 

General  Considerations 64 

The  Neurones 65 

Nerve-Fibres 68 

Medullated  Fibres 68 

Nonmedullated  Fibres 70 

Neuroglia 70 

Nerve-Trunks     71 

The  Ganglia   73 

Spinal  Ganglia   74 

Sympathetic  Ganglia  74 

Paraganglia 75 

Development  of  Nervous  Tissues  76 

Nerve-Terminations 79 

Sensory  Nerve-Endings 79 

Free  Sensory  Endings 79 

Encapsulated  Sensory  End- 
ings    80 

The  Tactile   Corpuscles  80 

The  End- Bulbs 81 

The  Genital  Corpuscles  8r 
The    Cylindrical    End- 
Bulbs S2 

The  Vater- Pacinian  Cor- 
puscles    82 

The  Golgi-Mazzoni  Cor- 
puscles    S3 

Neuromuscular  Endings  83 
Neurotendinous     End- 
ings      85 

Motor  Nerve- Endings 85 

Endings  in  Voluntary  Muscle  85 
Endings  in  Involuntary  Mus- 
cle    86 

Endings  in  Cardiac  Muscle. .  86 
v 


CONTENTS. 


THE    BLOOD-VASCULAR 
SYSTEM. 

General  Structure  of  Blood-\'essels .  .  87 

The  Arteries  89 

The  Veins 91 

The  Capillaries 92 

The  Blood  94 

The  Colored  Blood-Cells 94 

The  Colorless  Blood-Cells 96 

The  Blood-Plates 97 

Blood-Crystals 98 

Development  of  the  B]ood-\'ascular 

Tissues 99 

The  blood-Vessels 99 

The  Erythrocytes 99 

The  Colorless  Cells 100 

The  Heart  loi 

The  Endocardium  loi 

The  Myocardium 102 

The  Epicardium 103 

Development  of  the  Heart 104 

THE    LYMPHATIC    SYSTEM. 

General  Considerations  105 

The  Lymph-Spaces 106 

The  Lymph-Vessels   107 

L}Tnphoid  Tissue 108 

The  Simple  Lymph-Nodules    .  .  .  108 

The  Lymph-Nodes  109 

The  Hemolymph  Nodes    112 

Development  of  the  Lymphatic  Sys- 
tem    112 

The  Spleen    113 

Accessory  Spleens 116 

Vascular  Appendages 117 

The  Carotid  Bodies 117 

The  Coccygeal  Body 118 

MUCOUS   MEMBRANES    AND 
GLANDS. 

The  Mucous  Membranes 119 

The  Epithelium 120 

The  Tunica  Propria 120 

The  Submucous  Layer 121 

The  Blood-Vessels    121 

The  Lymphatics 121 

The  Nerves   121 

The  Glands  122 

General  Considerations  122 

Simple  Tubular  Glands 124 

Compound  Tubular  Glands   ....   124 

Tubo-Alveolar  Glands 124 

Serous  Glands    125 

Mucous  Glands 126 

Simple  Alveolar  Glands  ....  127 
Compound  Alveolar  Glands  127 
Development  of  Glands  ....   128 


THE  ALIMENTARY   CANAL. 

The  Oral  Cavity 129 

The  Mucous  Membrane 129 

The  Teeth 131 

The  Enamel 131 

The  Dentine   133 

The  Cementum 135 

Alveolar  Periosteum 135 

The  Pulp 136 

Development  of  the  Teeth   .  137 

The  Dental  Papilla  137 

Formation  of  Dentine  .  .    137 

The  Enamel-Organ 139 

Formation  of  Enamel  .     140 

The  Tooth-Sac  141 

Formation  of  Cementum    141 
The  Secondary  Dentition  ...    141 

The  Tongue    142 

The  Muscles    142 

The  Mucous  Membrane 142 

Papillary  Area    143 

Papillae  143 

Lymphoid  Area 145 

The  Lingual  Glands 145 

The  Oral  Glands    146 

The  Salivary  Glands 147 

The  Parotid  Gland   148 

The  Submaxillary  Gland 149 

The  Sublingual  Gland  149 

The  Palate 151 

The  Pharynx 152 

The  Mucous  Membrane 152 

The  Lj^mphoid  Tissue   153 

The  Faucial  Tonsil  153 

The  Phar^-ngeal  Tonsil   ....   154 

The  (Esophagus 154 

The  Mucous  Membrane 154 

The  Submucous  Layer 155 

The  Oesophageal  Glands  ...   155 

The  Muscular  Coat  156 

The  Stomach 157 

The  ^Mucous  Coat 157 

The  Gastric  Glands 158 

The  Submucous  Coat 160 

The  Muscular  Coat 161 

The  Serous  Coat   161 

The  Blood-Vessels 162 

The  Lymphatics 162 

The  Nerves  1 63 

The  Small  Intestine  163 

The  Mucous  Membrane   163 

The  Villi 164 

The  Plicse  Circulares 166 

The  Glands  166 

Brunner's  Glands 166 

Lieberkiihn's  Glands  . .  .    168 

The  Lymph-Nodules   168 

The  Solitary  Nodules  .  .   168 
Peyer's  Patches 169 


CONTENTS. 


The  Small  Intestine — Continued. 

The  Submucous  Coat 170 

The  Muscular  Coat 170 

The  Serous  Coat 170 

The  Blood-\'essels 170 

Lymphatics   171 

Xer\-es 171 

The  Large  Intestine   171 

The  Mucous  Membrane 171 

The  Submucous  Co;;t 172 

The  Muscular  Coat 172 

The  Serous  Coat 172 

The  Ileo-Ca;cal  Valve 173 

The  Vermiform  Appendix 173 

The  Peritoneum 175 

The  Liver 176 

The  Lobules 177 

The  Liver-Cells 17S 

The  Bile-Capillaries 179 

The  Biliary  Passages 181 

Interlobular  Bile-Ducts 181 

The  Hepatic  Duct 181 

The  Gail-Bladder 182 

The  Common  Bile-Duct  ....  183 

The  Blood-Vessels 183 

The  Lymphatics 1S3 

The  Nerves 183 

The  Pancreas 183 

The  Pancreatic  Duct 184 

The  Gland-Tissue  185 

The  Islands  of  Langerhans 1S5 

The  Blood-\'essels 186 

The  Lymphatics 1S6 

The  Nerves 1S6 

THE    ORGANS    OF    RESPIRA- 
TION. 

The  Larjnx 187 

The  Cartilages 187 

The  Mucous  Membrane 187 

The  Vocal  Folds 189 

The  Blood-Vessels 189 

The  Lymphatics 189 

The  Nerves 190 

The  Trachea  and  Bronchi 190 

The  Cartilage 190 

The  Mucous  Membrane 191 

The  Lungs 191 

The  Lobule  and  Lung-Units  ....  191 

The  Bronchioles 192 

The  Air-Spaces 194 

The  Blood-\'essels 195 

The  Lymphatics 196 

The  Nerves 196 

The  Pleurae 196 

The  Thyroid  Body 197 

The  Parathyroid  Bodies 199 

The  Thymus  Body 200 


THE    URINARY    ORGANS. 

The  Kidneys 205 

The  Architecture 205 

The  Kidney-Substance 207 

The  Course  of  the  Tubules  . .  207 

Details  of  the  Tubules 209 

The  Supporting  Tissue 213 

The  Blood-\'essels 213 

The  Lymphatics 215 

The  Nerves 215 

The  Renal  Ducts 215 

The  L'^reters 215 

The  Bladder 217 

The  Male  Urethra 219 

The  Female  Urethra 221 

The  Suprarenal  Bodies 222 

THE    MALE   REPRODUCTIVE 
ORGANS. 

The  Testicle  and  Its  Architecture.  .  .  227 

The  Testis — Spermatogenesis .  . .  229 

The  Epididymis 233 

Appendix  Testis 235 

Appendi.x  Epididymidis 235 

The  Paradidymis. 235 

Ductuli  Aberrantes 235 

The  Spermatic  Ducts  235 

The  \'as  Deferens 235 

The  Ampulla 236 

The  Ejaculatory  Duct 237 

The  Seminal  \'esicle 237 

The  Penis 239 

Corpora  Cavernosa 239 

Corpus  Spongiosum 239 

Blood-Vessels 239 

Nerves 241 

The  Prostate  Gland 241 

The  Bulbo-Urethral  Glands 244 

THE  FEMALE  REPRODUCTIVE 
ORGANS. 

The  Ovaries 245 

The  Follicles  and  Ova 245 

The  Human  Ovum 248 

Corpus  Luteum 249 

Rudimentary  Organs 252 

The  Epoophoron 252 

Gartner's  Duct 253 

The  Paroophoron 253 

Vesicular  Appendages 253 

The  Oviducts 254 

The  Uterus 255 

Endometrium 256 

Uterine  Glands 256 

Myometrium  257 

The  Vagina 259 


CONTENTS. 


The  External  Organs 260 

The  Labia 260 

The  Vestibule 261 

The  Paraurethral  Ducts 261 

The  Clitoris 262 

The  Bulbus  Vestibuli 262 

The  Glands  of  Bartholin 262 

The  Mammary  Glands 262 

Glandular  Tissue 262 

Excretory  Ducts 263 

The  Nipple 263 

Colostrum 264 

Milk 264 

THE    CENTRAL   NERVOUS 
SYSTEM. 

The  Spinal  Cord 266 

General  Considerations 267 

The  Gray  Matter 268 

Anterior  Horn  Cells 269 

Posterior  Horn  Cells 270 

Neuroglia  of  Gray  Matter 272 

The  Central  Canal 273 

The  White  Matter 273 

Neuroglia  of  White  Matter. .  274 

The  Fibre-Tracts 274 

The  Blood- Vessels 279 

The  Brain 279 

Outline  of  Gross  Anatomy 280 

The  Medulla  Oblongata 282 

The  Pons  Varolii 288 

The  Mid-Brain 291 

The  Cerebral  Peduncles. . .  .  292 

The  Tegmentum 292 

The  Cerebellum . .  294 

The  Cortex 294 

The  Internal  Nuclei 298 

The  Cerebral  Cortex 299 

The  Basal  Ganglia 304 

The  Pineal  Body 308 

The  Pituitary  Body 309 

The  Meninges 311 

The  Pacchonian  Bodies 314 

THE    SENSE    ORGANS. 

The  Skin 315 

The  Hairs 323 

Development  of  the  Hair  . .  .  328 

The  Nails 330 

The  Sebaceous  Glands 333 

The  Sweat-Glands 334 

The  Eye 337 

General  Considerations 337 

The  Fibrous  Tunic 338 

The  Sclera 338 

The  Cornea 339 

The  Sclero-Corneal  Junction  340 


The  ^ye— Continued. 

The  Vascular  Tunic  .^ 341 

The  Choroid 342 

The  Ciliary  Body 343 

The  Iris 345 

The  Nervous  Tunic 346 

The  Retina 346 

The  Macula  Lutea 351 

The  Ora  Serrata 352 

The  Optic  Nerves 353 

The  Crystalline  Lens 354 

The  Vitreous  Body 355 

The  Suspensory  Apparatus 356 

The  Eyelids  and  Conjunctiva 357 

The  Lachrymal  Apparatus 360 

The  Lachrymal  Gland 361 

The  Lachrymal  Canals 361 

The  Lachrymal  Sac 362 

The  Naso-Lachrymal  Duct 362 

The  Ear 362 

The  External  Ear 362 

The  Auricle    362 

The  External  Canal 363 

The  Middle  Ear 365 

The  Tympanic  Membrane. .  365 

The  Mucous  Membrane 366 

The  Ear  Ossicles 367 

The  Eustachian  Tube 367 

The  Mastoid  Cells 369 

The  Internal  Ear 369 

The  Bony  Labyrinth 369 

The  Membranous  Labyrinth  371 

Semicircular  Canals. ...  371 

Utricle  and  Saccule 371 

Cochlear  Duct 373 

Organ  of  Corti 376 

The  Nose 380 

The  Nasal  Mucous  Membrane  . .  380 

The  Olfactory  Region 381 

The  Respiratory  Region. . . .  382 

Jacobson's  Organ 384 

The  Accessory  Air-Spaces 384 

Organ  of  Taste. 385 

Taste-Buds 385 

APPENDIX— TECHNIQUE. 

Fixation  and  Preservation  of  Tissues  389 

Embedding 392 

Celloidin  Method 393 

Paraffin  Method 394 

.Section- Cutting 396 

Celloidin  Sections 396 

Paraffin  Sections 396 

Ribbon-Sections 397 

Staining  Methods 399 

Mounting  and  Finishing 403 

Special  Methods 405 

Injecting  Blood-vessels 408 


NORMAL  HISTOLOGY. 


THE    CELL 

All  animals  and  plants  are  composed  of  minute  structural  elements 
called  "cells."  With  the  exception  of  the  low  unicellular  forms,  in  which 
a  single  cell  constitutes  the  entire  organism,  the  fully  developed  animal  com- 
prises myriads  of  cells  arranged  as  the  tissues  composing  the  \-arious  parts  or 
organs. 

Notwithstanding  its  complexity,  the  body  of  even  the  highest  animal, 
man,  may  be  resohed  into  four  elementary  tissues — epithelial,  connective, 
vucscular  dL\\d  nervous — which  serve,  primarily,  for  the  purpose  of  protection, 
connection  and  support,  motion  and  control  respecti\ely.  Exery  tissue  con- 
sists of  two  parts,  the  cells  and  the  intercellular  substance.  L'pon  the  first 
of  these,  the  cells,  depend  the  vitality  and  growth  of  the  tissue;  while  the 
intercellular  substance  owes  its  production,  directly  or  indirectly,  to  the 
acti\'ity  of  the  cells. 

Ever\'  li\-ing  organism  is  derived  from  a  parent  cell,  the  ovum.  This 
element,  liberated  from  the  ovary  of  the  mother,  undergoes  certain  prepara- 
tory changes,  known  as  maturation,  and  then  unites  with  the  paternal 
germ-cell,  the  spermatic  filament  or  spe7'matozoon.  The  union  of  these  two 
sex-cells  results  in  fertilization  of  the  ovum.  The 
fertilized  ovum  immediately  di\ides  into  the  daughter  Nucleus 

cells,  each  of  which  gives  rise  to  two  new  elements;  ; 

each  of  these,  in  turn,  produces  two  descendants,  and  C^^^\ 

so  on.     As  the  result  of  segmentation,  as  this  cycle  of  ^^^'.?L  ';>^ 

repeated  division  is  termed,  a  numerous  progeny  of 
new  cells  arises  from  the  original  parent  cell.     The 

further  division  and  differentiation  of  the  segmenta-     Exopiasm  Endopiasm 

tion  cells  lead  to  the  formation  of  the  three  germ-layers  p^^  ^  —colorless  biood- 
— ihe  ectoderm,  the  7?iesoderm  a.nd  the  entoderm — from  corpuscle,  representing;  type 
which  the  definite  embryo  subsequently  is  evolved.        exi^bitPd"ifferentiatmn^fnto 

Notwithstanding  their  diversitv  of  form  and  size,     endopiasm  and  exopiasm.  x 
*        .  .  ■  .  2500. 

as  seen  in  the  adult  condition,  the  cells  of  the  animal 

body  possess  certain  features  in  common.  So  small  that  they  can  be  seen 
only  when  examined  with  the  microscope,  they  consist  of  a  minute  m.ass  of 
gelatinous  substance,  the  cell-body,  in  which  lies  embedded  a  still  smaller  round 
or  oval  body,  the  nnclens.  At  times  within  the  latter  a  distinct  dot,  the 
nucleolus,  is  seen.  The  original  conception  of  the  cell,  as  implied  by  its  name, 
was  that  of  a  minute  sac,  surrounded  bv  a  definite  membrane  or  cell-zcall,  filled 
with  fluid  and  enclosing  a  second  smaller  sac,  the  nucleus,  which,  in  turn, 
contained  a  third  saccule,  the  nucleolus.  Subsequent  study  established  the 
gelatinous,  not  fluid,  character  of  the  substance  of  the  cell-body,  or  cyto- 
plasm, and  the  frequent  absence  of  the  cell-wall  and  the  nucleolus.  The 
cell-body  and  the  nucleus  are,  therefore,  the  only  essential  parts  of  such 
structural  units  as  are  entitled  to  be  reg-arded  as  true  "cells" — retaining  and 


2  NORMAL  HISTOLOGY. 

using  this  misleading  term  in  its  accepted  but  not  literal  sense.  The  sub- 
stance of  the  entire  cell,  including  that  of  the  cell-body  and  of  the  nucleus, 
is  the  protoplasm,  the  cell  being  often  defined  as  "  a  minute  nucleated  particle 
of  protoplasm. ' ' 

THE    STRUCTURE   OF   THE   CELL. 

The  Cytoplasm. — The  translucent,  viscid  substance  forming  the  cell- 
body,  the  cytoplasm,  is  complex  in  both  its  chemical  and  structural  com- 
position. Chemically,  cytoplasm  consists  of  a  heterogeneous  mixture  of 
water,  salts  and  organic  compounds.  The  latter  are  grouped  under  the 
term   proteids,    which    are    complex    combinations    of    carbon,    hydrogen. 


>..■•■ 


Cytoplasm- 


V 


Karyosome 

Linin  thread 
Chromati 


-^-n-  ^    r>. 


'  Exoplasm 

-Endoplasm 

-Nuclear  membrane 
-Nucleus 


Centrosome  surrounded  by 
centrosphere 


Spongioplasm 
Hyaloplasm 

Metaplastic  inclusions 


Fig.  2. — Diagram  of  cell-structure.    In  the  upper  part  of  figure  the  granular  condition  of  the  cytoplasm 
is  represented;  in  the  lower  and  left  part,  the  reticular  condition. 

nitrogen  and  oxygen,  with  often  a  small  percentage  of  sulphur.  The  pro- 
teids  of  the  cytoplasm,  in  contrast  to  those  of  the  nucleus,  contain  little  or 
no  phosphorus. 

The  cytoplasm  by  no  means  always  presents  the  same  structural  appear- 
ance, since  its  constituents  are  subject  to  changes  in  their  condition  and 
arrangement  which  produce  corresponding  morphological  variations.  Thus, 
the  cytoplasm  may  be  devoid  of  recognizable  definite  structure  and  appear 
homogeneous;  at  other  times  it  may  present  aggregations  of  minute  spherical 
particles  and  then  be  described  as  g7'-anular,  or  where  the  minute  spheres  are 
larger  and  consist  of  fluid  substances  embedded  within  a  surrounding  denser 
material,  as  alveolar ;  or,  again,  and  most  frequently,  the  cytoplasm  contains 
a  meshwork  of  threads  or  fibrils,  more  or  less  conspicuous,  which  arrange- 
ment gives  rise  to  the  reticular  condition.  It  must  be  recognized,  therefore, 
that  the  structure  of  cytoplasm  is  not  to  be  regarded  as  immutable,  but 
on  the  contrary,  as  capable  of  undergoing  changes  which  render  it  probable 
that  a  cell  may  appear  during  one  stage  of  its  existence  as  granular  and  at  a 
later  period  as  reticular. 

Whatever  be  the  particular  phase  of  structure  exhibited  by  the  cell, 
histologists  are  agreed  that  the  cytoplasm  consists  of  two  substances—an 
active  and  a  passive  ;  while  both  must  be  regarded  as  living,  the  manifesta- 
tions of  contractility  are  probably  produced  by  the  former. 


STRUCTURE   OF   THE  CELL. 


Within  the  granular  cytoplasm  of  many  young  or  slightly  differentiated 
cells,  as  the  colorless  blood-cells,  the  active  substance  is  represented  by 
minute  spherical  particles,  or  tnicrosomes.  Often  these  are  not  uniformly 
distributed  in  the  cell-body,  a  narrow  peripheral  zone  of  variable  width  and 
firmer  consistence  being  almost  free  from  granules,  while  the  area  sur- 
rounding the  nucleus  is  densely  packed.  The  terms  exoplasni  and  endo- 
plasm  are  sometimes  employed  to  designate  the  homogeneous  peripheral 
and  granular  central  regions  of  the  cell-body  respectively.  Although  only 
very  exceptionally,  as  in  the  case  of  the  ovum,  is  the  animal  cell  possessed 
of  a  definite  limiting  membrane  or  cell-wall,  the  peripheral  layer  of  the 
cytoplasm  is  usually  of  greater  density.  Changes  in  the  surface  tension, 
which  is  thereby  reduced,  probably  account  for  the  alterations  in  form — the 
amoeboid  movements — and  similar  phenomena  often  regarded  as  ' '  vital ' ' 
manifestations. 

Since  a  more  or  less  pronounced  reticular  arrangement  of  the  active 
constituent  of  the  cytoplasm  is  widely  encountered  in  mature  cells,  this  con- 
dition may  serve  as  the  basis  of  the  description  of  the  morphology  of  the 
typical  cell.  Examination  of  suitably  prepared  preparations  with  adequate 
lenses  shows  the  cytoplasm  of  many  cells,  especially  the  highly  differen- 
tiated forms  of  glandular  epithelium,  to  contain  a  meshwork  composed  of 
delicate  threads  and  plates  of  the  more  active  substance,  the  spongiopiasm 
(also  called  mitome  or  \h&  filar  fnass).  The  spongioplastic  threads  contain 
rows  of  minute  granules,  the  microsomes,  either  scattered  or  closely  placed. 
Although  conspicuous  only  after  appropriate  staining,  threads  of  spongio- 
piasm may  at  times  be  seen  in  the  unstained  and  living  cell,  thereby  proving 
that  such  structural  details  are  not  artefacts  due  to  the  action  of  reagents 
upon  the  albuminous  substances  of  the  cytoplasm. 

The  interstices  of  the  meshwork  are  filled  with  a  clear,  more  or  less 
homogeneous  semifluid  substance  to  which  the  name  of  hyaloplasm  (also 
paraplasm,  paramitome  or  inter- 
filar  mass),  has  been  applied. 
Embedded  within  the  hyalo- 
plasm, a  variable  amount  of 
foreign  substances  is  frequently 
present.  These  include  parti- 
cles of  oil,  pigment,  secretory 
products  and  other  extraneous 
materials,  which,  while  of  pos- 
sible importance  in  fulfilling  the 
purpose  of  the  cell,  are  not 
among  its  essential  morpholog- 
ical constituents.  These  sub- 
stances, which  are  inert  and  take 
no  part  in  the  vital  activity  of 
the  cell,  are  termed  collectively 
metaplasm. 

The  Nucleus. — This,  the 
second  essential  constituent  of 
the  cell,  usually  appears  as  a  sharply  defined  spherical  or  ellipsoidal  body, 
which,  in  stained  preparations,  is  conspicuous  on  account  of  its  deeper  color. 
Since  the  nucleus  is  the  nutritive,  as  well  as  the  reproductive,  organ  of  the 
cell,  the  fact  that  this  part  of  the  cell  is  relatively  large  in  young  and  actively 
growing  elements  is  readily  explained.      While  in  a  general  way  the  nucleus 


#* 


^ 


Fig.  3. — Spermatogenic  cells,  showing  variations  in 
the  condition  and  the  arrangement  of  the  constituents  of 
the  cytoplasm  and  the  nucleus  ;  the  centrosomes  are  seen 
within  the  cytoplasm  close  to  the  nucleus.  A,  from  the 
guinea-pig,  X  1675  (Meves);  B,  from  the  cat,  ;<  680  (von 
Lenhossek). 


4  NORMAL  HISTOLOGY. 

corresponds  in  shape  with  the  form  of  the  cell,  being  oval  or  rod-like  in 
elongated  columnar  or  fusiform  cells,  and  compressed  or  flattened  in  plate- 
like elements,  its  outline  is  sometimes  very  irregular,  as  conspicuously  seen 
in  the  case  of  the  colorless  cells  (leucocytes)  of  the  blood.  At  times  the 
nucleus  is  capable  of  changing  its  form  or  even  position  independent  of 
the  surrounding  cytoplasm.  Except  during  certain  phases  of  division,  v/hen 
the  usual  demarcation  temporarily  disappears,  the  nucleus  is  sharply  defined 
from  the  cytoplasm  by  a  distinct  envelope,  the  nuclear  membrane.  The  latter 
encloses  the  substances  of  the  nucleus,  the  karyoplasm,  which  structurally 
resembles  the  cytoplasm  in  being  composed  of  two  parts — an  irregular 
reticulum  of  mtclear  fibrils  and  an  intervening  semifluid  miclear  matrix. 

The  nuclear  fibrils,  when  examined  under  high  magnification  after 
appropriate  treatment  with  suitable  stains,  such  as  hematoxylin,  safranin 
and  other  basic  dyes,  are  shown  to  consist  of  minute  irregular  masses  of  a 
deeply  colored  substance,  appropriately  called  chromatin,  in  recognition  of 
its  great  affinity  for  certain  stains.  The  chromatin  particles  are  supported 
upon  or  wdthin  delicate  inconspicuous  and  almost  colorless  threads  of  linin. 
The  latter  forms  the  basis  of  the  supporting  framework  of  the  nuclear  fibrils, 
in  which  the  chromatin  is  so  conspicuous  by  reason  of  its  capacity  for  stain- 
ing. The  individual  masses  of  chromatin  vary  greatly  in  form,  often  being 
irregular,  and  at  other  times  thread-like  or  beaded.  Not  infrequently  the 
chromatin  presents  spherical  aggregations  which  appear  as  deeply  stained 
nodules  attached  to  the  nuclear  fibrils.  These  constitute  the  false  nucleoli, 
or  karyosomes,  as  distinguished  from  the  true  nucleolus,  which  is  usually 
present  within  the  karyoplasm.  Chemically,  chromatin,  the  most  important 
part  of  the  nucleus,  contains  miclein,  a  compound  rich  in  phosphorus. 

The  nuclear  matrix,  the  fluid  or  semifluid  substance  which  occupies 
the  spaces  between  the  nuclear  fibrils,  possesses  an  exceedingly  weak  affinity 
for  the  staining  reagents  employed  to  color  the  chromatin.  It  usually  appears, 
therefore,  clear  and  untinted,  and  contains  a  substance  described  as  paralinin. 
The  nucleolus,  or  plasmosome,  ordinarily  appears  as  a  small  spherical 
body,  sometimes  multiple,  lying  among,  but  unattached  to,  the  nuclear 
fibres.  In  stained  tissues  its  color  varies,  sometimes  resembling  that  of  the 
chromatin,  although  less  intense,  but  usually  presenting  a  different  tint, 
since  it  responds  readily  to  dyes  which,  like  eosin  or  acid  fuchsin,  particu- 
larly affect  the  linin  and  cytoplasm.  Concerning  the  nature,  purpose  and 
function  of  the  nucleolus  much  uncertainty  exists.     According  to  certain 

authorities  these  bodies  are  to  be  regarded 
as  storehouses  of  substances  which  are  used 
in  forming  the  chromatin  segments  during 
cell-division,  while  other  cytologists  attribute 
■      '     '  '  to  the  nucleolus  a  passive  role,  even  holding 

it  to   be  a  by-product,   which,   at  least   in 
some   cases,  is   cast  out  from    the  nucleus 
into  the  cytoplasm,  where  it  may  disappear. 
P  ^        ,,  .,,  1-1     11     1,  The  nucleolus  is  credited  with  containing  a 

Fig.  4. — Human   epitnelial  cells  snow-  .  .  * 

ing  paired  ctiitrosomes  (c,  c)\  A,  irom      peculiar  substaucc  knowu  ^s pv renin. 

gastric  glands ;  ^,  from  duodenal  glands.  /-rAi.      r->        j. t      '  uv    „  j.     tt- 

X  690.   (A-,  w.  zimmermann.)  The  Centrosome. — In  addition  to  the 

.  parts  already  described,  many  animal  cells 
contain  a  minute  body,  the  centrosome,  which  probably  plays  an  important 
role  during  division  and,  in  a  lesser  degree,  during  other  phases  of  cellular 
activity.  Ordinarily  the  centrosome  escapes  attention  because,  on  account 
of  its  minute  size  and  variable  staining  affinity,  it  is  with  difficulty  distin- 


VITAL   MANIFESTATIONS.  5 

guished  from  the  surrounding-  granules.  Its  usual  position  is  within  the 
cytoplasm,  but  the  exact  location  seems  to  depend  upon  the  focus  of 
greatest  motor  activity;  thus,  in  a  dividing  element,  the  centrosome  lies 
immediately  related  to  the  actively  changing  nucleus,  while  within  ciliated 
epithelium  it  is  found  closely  associated  with  the  contractile  filaments  con- 
nected with  the  hair-like  appendages.  In  recognition  of  the  intimate 
relations  between  this  minute  body  and  the  motor  changes  affecting  the  cell, 
the  centrosome  may  be  regarded  physiologically  as  its  dynamic  centre. 
The  centrosome,  often  represented  by  a  pair  of  minute  granules  (diphsome), 
is  frequently  surrounded  by  a  clear  area  or  halo,  the  centrosphere.  As  seen 
in  certain  invertebrate  cells,  the  centrosome  is  resolvable  into  a  minute 
granule,  the  centric/e,  embedded  within  a.  substance  known  as  the  cefitro- 
plasvi. 

VITAL    PHENOMENA. 

The  vital  manifestations  of  the  cell  include  those  complex  physico-chem- 
ical changes  which  occur  during  the  life  of  the  cell  in  the  performance  of  its 
appointed  work.  They  embrace  metabolism,  growth,  reproduction  and 
irritability. 

Metabolism,  the  most  distinctive  characteristic  of  living  matter,  is  that 
process  whereby  protoplasm  selects  from  the  heterogeneous  materials  of  food 
those  particular  substances  which  are  suitable  for  its  nutrition  and  converts 
them  into  its  own  substance.  Metabolism  is  of  two  kinds — constructive  and 
destructive.  Constrtictive  metabolism,  or  anabolism,  is  the  process  by  which 
the  cell  converts  the  simpler  compounds  into  organic  substances  of  great 
chemical  complexity.  Destructive  metabolism,  or  katabolism,  is  the  process 
by  which  the  cell  breaks  up  the  complex  substances  resulting  from  construc- 
tive metabolism  into  simpler  compounds.  Vegetal  cells  possess  the  power 
of  constructive  metabolism  in  a  conspicuous  degree  and  from  the  simpler 
substances,  such  as  water,  carbon  dioxide  and  inorganic  salts,  prepare  food- 
material  for  the  nutritive  and  katabolic  processes  which  especially  distinguish 
animal  cells.  The  latter  are  dependent,  directly  or  indirectly,  upon  the  veg- 
etal cells  for  their  nutritive  materials. 

Growth,  the  natural  sequel  of  the  nutritive  changes  effected  by  metab- 
olism, may  be  unrestricted  and  equal  in  all  directions,  resulting  in  uniform 
expansion  of  the  spherical  cell,  as  illustrated  in  the  growth  of  the  ovum  in 
attaining  its  mature  condition.  Such  unrestricted  growth,  however,  is 
exceptional,  since  cells  are  usually  more  or  less  intimately  related  to  other 
structural  elements  by  which  their  increase  in  size  is  modified  so  as  to  be 
limited  to  certain  directions.  Such  limitation  and  influence  result  in  unequal 
gi'owth,  a  force  of  great  potency  in  bringing  about  the  differentiation  and 
specialization  of  cells,  and,  secondarily,  of  organs  and  entire  parts  of  the  body. 
Familiar  examples  of  the  results  of  unequal  growth  are  seen  in  the  columnar 
cells  of  epithelium,  the  fibres  of  muscular  tissue,  and  the  neurones  of  the 
nervous  system. 

Reproduction  may  be  regarded  as  the  culminating  vital  manifestation 
in  the  life-cycle  of  the  cell,  since  by  this  process  the  parent  cell  surrenders 
its  individuality  and  continues  its  life  in  the  existence  of  its  offspring.  Cell- 
reproduction  occurs  by  two  methods — the  indirect  or  mitotic  and  the  direct 
or  amitotic.  The  first  of  these,  involving  the  complicated  cycle  of  nuclear 
changes  known  as  mitosis  or  karyokinesis,  is  the  usual  method;  the  second 
and  simpler  process  of  direct  division  is  exceptional  and  frequently  associated 
with  conditions  of  impaired  vital  vigor. 


6  NORMAL   HISTOLOGY. 

Irritability  is  that  property  of  living  matter  by  virtue  of  which  the  cell 
exhibits  changes  in  its  form  and  intimate  constitution  in  response  to  external 
impressions.  The  latter  may  originate  in  mechanical,  thermal,  electrical  or 
chemical  stimuli  to  which  the  protoplasm  of  e\'en  the  lowest  organisms 
responds;  or  they  may  be  produced  in  consequence  of  obscure  and  subtle 
changes  occurring  within  the  protoplasm  of  neighboring  cells,  as  illustrated 
by  the  reaction  of  one  neurone  in  response  to  the  stimuli  transmitted  from 
other  nervous  elements. 

CELL   DIVISION. 

With  the  exception  of  the  unusual  cases  in  which  division  takes  place 
by  the  direct  or  amitotic  method,  the  production  of  new  generations  of  cells 
of  all  kinds  is  accomplished  by  a  complicated  series  of  changes,  collectively 


Fig.  5. — Diagram  of  mitosis.  A,  resting  stage,  chromatin  irregularly  distributed  in  nuclear  reticu- 
lum ;  a,  centrosphere  containing  double  centrosome ;  n,  nucleolus.  B,  chromatin  arranged  as  close 
spirem  ;  c.  c,  centrosomes  surrounded  by  achromatic  radial  striatioiis.  C,  stage  of  loose  spirem,  achro- 
matic figure  forming  amphiaster  (amp).  D,  chromatin  broken  into  chromosomes;  nucleolus  has  dis- 
appeared, nuclear  membrane  fading;  amphiaster  consists  of  two  asters  (a,  a)  surrounding  the  separating 
centrosomes,  connected  by  the  spindle  [s).  E,  longitudinal  cleavage  of  the  chromosomes  which  are 
arranged  around  the  polar  field  (/)  occupied  by  the  spindle.  F,  migration  of  chromatic  segments 
towards  new  nuclei,  as  established  by  centrosomes  (c,  c\\  e p,  equatorial  plate  formed  by  intermingling 
segments.  G,  separating  groups  of  daughter  chromosomes  \d,  d)  united  by  connecting  threads  (ct).  H, 
daughter  chromosomes  id,  d)  becoming  arranged  around  daughter  centrosomes  which  have  already 
divided  ;  C.  C,  beginning  cleavage  of  c>-toplasm  across  plane  of  equatorial  spindle.  /,  completed  daughter 
nuclei    D,  D)\  c>-toplasm  almost  divided  into  two  new  cells.     {Modified  from  Wilson.) 

known  as  mitosis  or  karyokinesis,  especially  affecting  the  nucleus.  In 
addition  to  presiding  over  the  nutritive  changes  within  the  cell,  the  nucleus 
is  particularly  concerned  in  the  process  of  reproduction;  further,  of  the 
various  morphological  constituents  of  the  nucleus,  the  chromatin  displays 
the  most  active  change,  since  this  substance  is  the  vehicle  by  which  the  char- 
acteristics of  the  parent  cell  are  transmitted  to  the  new  elements.     .So  essen- 


CELL-DIVISION.  7 

tial  is  this  substance  for  the  perpetuation  of  the  specific  character  of  the  cell, 
that  the  entire  complex  mitotic  cycle  has  for  its  primary  purpose  the  insur- 
ance of  the  equal  division  of  the  chromatin  of  the  mother  cell  between  the 
two  new  nuclei.  Such  impartial  distribution  of  the  maternal  chromatin  takes 
place  irrespective  of  any,  or  even  very  great,  dissimilarity  in  the  size  of  the 
daughter  cells,  the  smaller  receiving  one-half,  or  exactly  the  same  amount 
of  chromatin  as  the  larger. 

Mitotic  Division. — The  details  of  mitosis  or  karyokinesis  include  a 
series  of  changes  involving  the  nucleus,  the  centrosome  and  the  cytoplasm. 
These  changes  are  grouped  conveniently  into  four  stages:  (i)  the  Pro- 
phases, or  preparatory  changes;  (2)  the  Metap/iase,  during  which  the  mass 
of  maternal  chromatin  is  equally  divided;  (3)  the  Anaphases,  in  which  the 
chromatin  is  distributed  to  the  new  nuclei;  and  (4)   the  Telophases,  during 


M 


f^^ 


Fig.  6. — Chromatic  figures  in  dividing  cells  from  epidermis  of  salamander  embryo.  X  850.  A^  rest- 
ing stage;  .5,  close  spireme;  C,  loose  spireme;  Z),  chromosomes  ("wreath"),  seen  from  surface;  E, 
similar  stage,  seen  in  profile ;  F,  longitudinal  cleavage  of  chromosomes  ;  G,  beginning  migration  of  seg- 
ments towards  centrosomes  ;  H,  separating  groups  of  daughter  segments ;  /,  daughter  groups  attracted 
towards  poles  of  new  nuclei,  cytoplasm  exhibits  beginning  cleavage. 

which  the  cytoplasm  of  the  mother  cell  undergoes  division  and  the  daughter 
cells  are  completed. 

Mitosis  includes  two  distinct  but  closely  associated  and  coincident  series 
of  phenomena,  the  one  involving  the  chromatin  and  the  other  the  centro- 
some and  the  linin.  While  as  a  matter  of  convenience  these  two  sets  of 
changes  are  described  separately,  it  must  be  understood  that  they  take  place 
simultaneously  and  in  coordination.  The  purpose  of  the  changes  affecting 
the  chromatin  is  the  accurate  and  equal  division  of  this  substance  by  the 
longitudinal  cleavage  of  the  chromatin  segments.  The  object  of  the  activity 
of  the  centrosomes  and  the  linin  is  to  supply  the  requisite  energy  and  guid- 


8  NORMAL    HISTOLOGY. 

ance  by  which  the  chromatin  segments  are  directed  to  the  new  nuclei  in 
process  of  formation,  each  daughter  cell  being  insured  in  this  manner  one- 
half  of  the  maternal  chromatin. 

The  Prophases,  or  preparatory  stages,  include  a  series  of  changes 
which  involve  the  nuclear  substances  and  the  centrosome,  and  result  in  the 
production  of  the  mitotic  figure.  The  latter  consists  of  two  parts,  (i)  the 
deeply  staining  chromatin  filaments  and  (2)  the  achromatic  figure,  which 
colors  only  very  slightly  if  at  all.  The  chromatin,  which  before  division 
begins  is  disposed  along  the  irregular  nuclear  fibrils,  loses  its  reticular 
arrangement  and,  increasing  in  amount  as  well  as  in  its  staining  affinities, 
becomes  transformed  into  a  closely  convoluted  thread  or  threads,  constitu- 
ting the  ' '  close  skein. "  The  filaments  composing  the  latter  soon  shorten  and 
thicken  to  form  the  "loose  skein."  The  skein,  or  spireme,  may  consist  of  a 
single  continuous  filament,  or  it  may  be  formed  of  a  number  of  separate 
threads.  Sooner  or  later  the  skein  breaks  up  transversely  into  a  number  of 
segments  or  chromosomes,  which  appear  as  deeply  stained  curved  or  straight 
rods.  A  very  important,  as  well  as  remarkable,  fact  regarding  the  chromo- 
somes, is  their  numerical  constancy,  since  the  cells  of  every  animal  and  plant 
always  possess  a  definite  number  of  chromosomes,  corresponding  to  the 
quota  for  that  particular  species;  further,  in  all  the  higher  animals  the 
number  is  even,  in  man  being  probably  twenty-four.  During  these  changes 
affecting  the  chromatin,  the  micleolus,  or  plasmosome,  disappears  and, 
probably,  takes  no  active  part  in  mitosis.  The  nuclear  membrane  likewise 
fades  away  during  the  prophases,  the  chromatic  segments  now  lying 
unenclosed  within  the  cell,  in  which  the  cytoplasm  and  nuclear  matrix  are 
continuous. 

Coincident  with  the  foregoing  changes,  the  centrosome,  which  by  this 
time  has  become  double,  is  closely  associated  with  the  achromatic  figure. 
A  delicate  radial  striation  appears  around  each  centrosome,  thereby  pro- 
ducing an  arrangement  resembling  stars  or  asters.  The  centrosomes  early 
manifest  a  disposition  to  separate  towards  opposite  poles  of  the  cell,  this 
migration  resulting  in  a  corresponding  migration  of  the  asters.  In  conse- 
quence of  these  changes,  the  retreating  centrosomes  become  the  foci  of  two 
systems  of  radial  striation  which  meet  and  together  form  an  achromatic 
figure  known  as  the  amphiaster.  The  latter  consists  of  the  two  asters  and 
the  intervening  spindle.  There  seems  little  doubt  that  the  centrosomes  play 
an  important  role  in  establishing  foci  towards  which  the  chromosomes  for  the 
new  nuclei  become  attracted.  Subsequently  the  nuclear  spindle,  which 
originates  from  the  amphiaster,  often  occupies  the  periphery  of  the  nucleus, 
whose  limiting  membrane  by  this  time  has  disappeared.  The  delicate 
threads  of  linin  composing  the  nuclear  spindle  extend  within  an  area,  the 
polar  field,  around  which  the  chromosomes  become  grouped.  The  chromo- 
somes, which  meanwhile  have  arisen  by  transverse  division  of  the  chromatin 
threads  composing  the  loose  skein,  appear  as  V-shaped  segments,  the 
closed  ends  of  the  loops  being  directed  towards  the  polar  field  which  they 
encircle. 

The  Metaphase  includes  the  most  important  detail  of  mitosis — namely, 
the  lo7igitudinal  cleavage  of  the  chromosomes,  whereby  the  number  of  the 
latter  is  doubled  and  the  chromatin  is  equally  divided.  The  cleavage  and 
division  are  the  first  steps  towards  the  actual  apportionment  of  the  chroma- 
tin between  the  new  nuclei,  each  of  which  receives  not  only  exactly  one-half 
of  the  chromatin,  but  the  full  quota  of  chromosomes,  and  this  irrespective  of 
even  marked  inequality  in  the  size  of  the  new  daughter  cells. 


CELL-DIVISION.  9 

The  notable  exception  to  the  constancy  of  the  numerical  quota  of 
the  chromosomes  presented  by  the  germ-cells  should  be  mentioned.  Since 
the  chromosomes  of  the  fertilized  ovum  are  derived  from  the  chromatin 
contributed  equally  by  the  paternal  and  maternal  germ-cells — the  spermato- 
zoon and  the  o\um — it  is  evident  that  unless  the  number  of  chromosomes 
from  each  parent  be  only  one-half  the  usual  number  for  the  species,  the 
segmentation  nucleus  and  the  succeeding  cells  would  contain  twice  the 
normal  quota  of  chromosomes.  In  order  to  prevent  such  redundancy, 
during  the  development  of  the  spermatic  cells,  on  the  one  hand,  and  the 
maturation  of  the  ovum  on  the  other,  reduction  of  the  chromosomes  to 
one-half  the  usual  number  actually  takes  place.  The  details  by  which  this 
reduction  is  accomplished  vary  in  different  classes  of  animals  ;  but,  whatever 
be  the  method,  the  result  is  to  reduce  the  number  of  chromatin-masses 
one-half.  This  usually  occurs  just  before  the  first  of  the  divisions,  pro- 
ducing the  capable  germ-cells.  The  full  quota  for  the  species  is  restored 
to  the  segmentation  nucleus  and  its  descendants  by  the  subsequent  ad- 
dition of  the  reduced  contingents  of  the  two  germ- cells  when  fertilization 
occurs. 

Meanwhile  the  centrosomes  have  continued  to  migrate  towards  the 
opposite  poles  of  the  dividing  cell,  where  each  forms  the  centre  of  the  astral 
radiation  that  marks  either  pole  of  the  amphiaster.  The  purpose  of  the 
achromatic  figure  is  to  guide  the  longitudinally  cleft  chromosomes  towards 
the  new  nuclei  during  the  succeeding  changes. 

The  Anaphases  accomplish  the  migration  of  the  chromosomes,  each 
pair  of  sister  segments  contributing  one  unit  to  each  of  the  two  groups  of 
chromosomes  that  are  passing  towards  the  poles  of  the  achromatic  spindle. 
In  this  manner  each  new  nucleus  receives  not  only  one-half  of  the  chromatin 
of  the  mother  nucleus,  but  also  the  same  number  of  chromosomes  that  orig- 
inally existed  within  the  mother  cell,  the  numerical  constancy  of  the  particu- 
lar species  being  thus  maintained. 

In  the  beginning  of  their  passage  tow^ards  the  poles  of  the  achromatic 
figure,  the  migrating  chromatic  segments,  attracted  along  the  linin  threads, 
for  a  time  form  a  compact  group  about  the  equator  of  the  spindle.  As  the 
receding  segments  pass  towards  their  respective  poles,  the  opposed  ends  of 
the  separating  chromosomes  are  united  by  intervening  achromatic  threads, 
the  connecting  fibres.  Sometimes  the  latter  exhibit  a  linear  series  of  thick- 
enings, known  as  the  cell-plate-ox  mid-body.  The  migration  of  the  chromo- 
somes establishes  the  essential  features  of  the  division  of  the  nucleus,  since 
the  subsequent  changes  are  only  repetitions,  in  reverse  order,  of  the  details 
of  the  prophases. 

The  Telophases,  in  addition  to  the  final  stages  in  the  rearrangement 
of  the  chromatic  segments  of  the  new  nuclei,  including  the  appearance  of  the 
daughter  skeins,  of  the  new  nuclear  membranes  and  of  the  nucleoli,  inaugu- 
rate the  participation  of  the  cytoplasm  in  the  formation  of  the  new  cells. 
During  these  final  stages  of  mitosis,  the  cell-body  becomes  constricted  and 
then  divides  into  two,  the  plane  of  division  coinciding  with  the  equator  of 
the  nuclear  spindle.  Each  of  the  resulting  masses  of  cytoplasm  invests  a 
new  nucleus  and  receives  one-half  of  the  achromatic  figure,  consisting  of  a 
half-spindle  and  one  of  the  asters  wath  a  centrosome.  The  new  cell,  now 
possessing  all  the  constituents  of  the  parent  element,  usually  acquires  the 
morphological  characteristics  of  its  ancestor  and  passes  into  a  condition  of 
comparative  rest,  until  called  upon,  in  its  turn,  to  undergo  division  and  enter 
upon  the  complicated  cycle  of  mitosis. 


lO 


NORMAL    HISTOLOGY. 


SYNOPSIS   OF  MITOTIC   DIVISION. 
I.     Prophases : 

A.  Changes  within  the  nucleus  :  Chromatic  figure. 

Chromatin  loses  reticular  arrangement. 
Close  skein. 
Loose  skein. 

Disappearance  of  nucleolus. 
Division  of  skein  into  chromosomes. 
Chromosomes  grouped  around  polar  field. 
Disappearance  of  nuclear  membrane. 

B.  Changes  within  the  cytoplasm :  Achromatic  figure. 

Division  of  centrosome. 

Appearance  of  asters. 

Migration  of  centrosomes. 

Formation  of  amphiaster. 

Appearance  of  nuclear  spindle  and  polar  field. 

II.     Metaphase : 

Longitudinal  cleavage  of  chromosomes. 

III.  Anaphases : 

Rearrangement  of  chromosomes  into  two  groups. 
Migration  of  groups  towards  poles  of  amphiaster. 
Appearance  of  connecting  fibres  between  receding  groups. 
Construction  of  daughter  nuclei. 

IV.  Telophases : 

Constriction  of  cell-body  at  right  angles  to  axis  of  spindle. 
Chromosomes  rearranged  as  daughter  skeins. 
Appearance  of  nuclear  membranes. 
Appearance  of  nucleoli. 
Complete  division  of  cell-body. 
Daughter  nuclei  assume  vegetative  condition. 
Achromatic  striation  usually  disappears. 
Centrosomes,  single  or  double,  lie  beside  new  nuclei. 

Amitotic    Division. — The   occurrence  of   cell   reproduction  without 
the  complex  cycle  of  karyokinetic  changes,  is  known  as  amitotic  or  direct 

diznsion.  This  process  takes  place  as  an  ex- 
ceptional method  in  the  reproduction  of  the 
simplest  forms  of  life,  and  in  the  multiplication 
of  cells  within  pathological  growths  or  tissues 
of  a  transient  nature,  as  the  foetal  envelopes. 

The  essential  difference  between  the  ami- 
totic and  the  usual  method  of  division  lies  in 
the  fact  that,  while  in  the  latter  the  chromatin 
is  equally  divided  and  the  number  of  chro- 
mosomes carefully  maintained,  in  the  direct 
method  the  nucleus  remains  passive  and  suffers 
cleavage  of  its  total  mass,  but  not  of  its  indi- 
vidual components,  by  constriction  or  fission. 
Neither  the  chromatic  nor  the  achromatic 
figure  is  produced,  the  activity  of  the  centrosome,  when  exhibited,  being 
uncertain  and  perhaps  direcdy  expended  in  .effecting  division  of  the  cyto- 
plasm and,  incidentally,  of  the  nucleus.      In  many  cases  amitotic  division  of 


Fig.  7. — Decidual  cells  exhibiting 
amitotic  division  of  nucleus  [A-D)\ 
in  E  irregular  mitosis  has  occurred. 
X  .350. 


THE   GERM-CELLS. 


1 1 


the  nucleus  is  not  accompanied  by  cleavage  of  the  cytoplasm,  such  processes 
resulting-  in  the  production  of  multinuclear  and  aberrant  forms  of  cells. 
In  general,  it  may  be  assumed  that  cells  which  undergo  amitotic  division  are 
destined  to  suffer  premature  degeneration,  since  such  elements  subserve 
special  purposes  and  are  usually  incapable  of  perpetuating  their  kind  by 
normal  reproduction.  Flemming  has  pointed  out  that  leucocytes  which 
arise  by  direct  division,  and  therefore  deviate  from  the  usual  mode  of  origin 
of  these  elements,  are  doomed  to  early  death.  Among  the  higher  animals, 
amitotic  division  must  be  regarded,  probably,  as  a  secondary  process. 

ORIGIN  AND  DIFFERENTIATION  OF  THE  CELLS. 

The  body,  with  all  its  complex  details,  is  the  product  of  the  differen- 
tiation and  specialization  of  cells  which  are  the  descendants  of  the  fertilized 
ovum.      The  latter  represents  the  two  parents,  since  the  chromatin  of  the 


Corona  radiata 


—  Zona  pellucida 


Germinal  vesicle  (nu- 
cleus) containing  ger- 
minal spot  (nucleolus) 

Zone  rich  in 
deutoplasm 

Zone  poor  in 
deutoplasm 


Fig.  8. — Human  ovum  from  ripe  Graafian  follicle.     X  160.     (Nagel.) 


segmentation  nucleus  is  contributed  equally  by  the  germ-cells,  the  sperma- 
tozoon and  the  ovum. 

The  ovum  is  formed  within  the  female  sexual  gland,  the  ovary,  where 
it  passes  through  all  stages  of  development,  from  immaturity  to  maturation, 
until  finally  liberated  by  rupture  of  the  ovarian  tissue.  As  a  cell,  the  ovum 
is  interesting,  since  it  possesses  all  parts  of  the  typical  cell,  including  a  cell- 
wall.  These  parts  have  long  been  designated  by  special  names ;  thus,  in  the 
nomenclature  of  the  egg,  the  cytoplasm  is  called  the  vitelhis  or  yolk,  the 
nucleus  the  germinal  vesicle,  the  nucleolus  the  germinal  spot,  and  the  cell- 
wall  the  oolemma  or  vitelline  membrane.  While  the  ova  of  birds  and  rep- 
tiles are  often  of  huge  size,  the  yolk  of  the  hen's  egg  corresponding  to  a 
single  cell,  the  true  ovum,  the  mammalian  ova  are  much  smaller  and  barely 
visible  with  the  unaided  eye.  The  hnman  ovum,  when  discharged  from  the 
ovary,  is  about  .2  millimeter  in  diameter,  spherical  in  form  and  composed 
of  cytoplasm  containing  innumerable  yolk-granules.  The  latter,  the  repre- 
sentatives of  the  abundant  masses  of  nutriti\'e  material  or  deutoplasm  stored 
as  the  food-yolk  in  the  bird's  egg,  are  especially  numerous  in  the  vicinity  of 


12 


NORMAL    HISTOLOGY. 


Head 

Neck 
Connect- 
ing piece 


Tail 


the  nucleus.  Towards  the  periphery  of  the  cell  they  are  nearly  wanting,  a 
narrow  zone  of  almost  homogeneous  cytoplasm  lying  immediately  beneath 
the  delicate  vitelline  membrane.  The  liberated  ovum  is  surrounded  by  a 
protecting  membrane,  the  zo7ia  pellucida,  which  some- 
times exhibits  a  faint  radial  striation.  This  envelope 
must  not  be  confounded  with  the  vitelline  membrane, 
since  it  is  not  strictly  a  part  of  the  ovum,  but  a  product 
of  the  surrounding  epithelial  cells  lining  the  little  sac,  the 
Graafia7i  follicle ^  enclosing  the  &^^  while  within  the  ovary. 
The  large  eccentric  spherical  nucleus,  the  germinal  vesicle, 
is  about  37  p}  in  diameter  and  surrounded  by  a  distinct 
nuclear  membrane.  Within  the  germinal  vesicle  are 
found  the  usual  constituents  of  the  nucleus,  including 
the  all-important  chromatin  fibrils,  nuclear  matrix  and 
nucleolus.  The  latter,  the  germinal  spot,  is  distinct  and 
measures  about  5  p.  in  diameter. 

The  spermatozoon,  the  male  germ-cell,  is  produced 
by  the  specialization  of  epithelial  cells  lining  the  semi- 
niferous tubules  within  the  testis.  The  human  sperma- 
tozoon consists  of  three  chief  parts — the  ovoid  head, 
middle-piece,  which  includes  the  slightly  constricted  neck 
and  the  connecting  piece,  and  the  attenuated  and  greatly 
extended  tail.  Although  the  entire  length  of  the  sper- 
matic element  is  about  50  ,«,  the  head  measures  only 
about  5  p. ;  the  male  germ-cell,  therefore,  is  much  smaller 
than  the  ovum.  The  head  and  the  neck  are  the  most 
important  parts,  since  they  contain  respectively  the  chro- 
matin and  the  centrosome  of  the  cells,  the  spermatids, 
from  which  the  spermatozoa  are  directly  derived. 

The  centrosome  is  represented  by  two  minute  spher- 
ical bodies,  the  neck-g7^anides,  which  lie  in  the  neck 
immediately  beneath  the  head  and  at  the  anterior  ex- 
tremity of  the  connecting  piece.  The  axial Jibi^e  extends 
throughout  the  spermatozoon  from  the  neck  to  the  tip 
of  the  tail,  ending  as  an  attenuated  thread,  the  terminal 
Jilainent.  The  tail  corresponds  to  a  flagellum  and  serves  the  purpose  of 
propulsion  alone,  taking  no  part  in  the  important  changes  within  the  ovum 
incident  to  fertilization,  during  which  the  head  and  middle-piece  enter  the 
substance  of  the  ^gg. 

Immediately  following  the  construction  of  a  new  nucleus  from  the  chro- 
matin contributed  by  the  two  parental  germ-cells,  the  fertilized  ovum  enters 
upon  a  cycle  of  repeated  division.  As  the  result  of  this  process,  known  as 
segnientatio7i,  in  which  the  new  cells  arise  by  mitotic  division,  a  spherical 
mass  of  young  cells,  the  morula,  is  produced.  This  mass,  at  first  solid, 
soon  acquires  a  central  cavity  filled  with  fluid  and  is  converted  into  a  hollow 
sphere,  known  as  the  blastodermic  vesicle.  The  wall  of  this  sac  consists  of 
a  single  layer  of  cells,  except  at  one  place  where  a  small  mass  of  cells  is 
attached  to  the  inner  surface.  The  outer  or  covering  layer  of  cells  is  the 
trophoblast ;  the  group  of  cells  attached  to  the  inner  surface  of  the  tropho- 
blast  is  the  hiner  cell-mass.      Corresponding  to  the  position  of  the  latter, 


Terminal 
filament 


Fig.  9. — Diagram  of 
human  spermatozoon; 
c,  neck-granules,  rep- 
resenting the  centro- 
some; 6,  axial  fibre. 
X  1800.     {Meves). 


^  The  sizes  of  microscopic  objects  are  usually  expressed   in  thousandths  of  a 
millimeter,  represented  by  the  letter  [i;  i  /i  (micron)  =  .001  mm. 


THE   GERM-LAYERS. 


13 


the  surface  of  the  blastodermic  vesicle  presents  an  opaque  circular  field,  the 
embryonic  area,  so  called  from  the  fact  that  \\'ithin  this  area  the  first  traces 
of  the  future  embryo  appear. 

In  consequence  of  further  growth  and  differentiation  of  the  inner  cell- 
mass,  the  latter  gives  rise  to  two  sheets  of  cells,  the  ectoderm  and  the 
entoderm.  The  first  of  these  is  continuous  with  the  trophoblast  and,  in  con- 
junction with  the  latter,  completes  the  outer  layer  of  the  blastodermic  vesi- 


FlG.  10. — Early  stages  of  segmentation  as  seen  in  sections  of  ova  of  mouse.  X  450-  iSobotia.) 
A-D  show  the  rearrangement  of  the  chromosomes  contributed  by  the  male  (w)  and  female  (/)  germ- 
cells  as  preparatory  to  the  first  cleavage  of  the  fertilized  ovum  ;  p,  p,  polar  bodies  ;  e  p,  stage  of  equatorial 
plate ;  a,  b,  daughter  groups  of  chromosomes.  E,  F,  the  daughter  cells  arising  from  first  cleavage.  G, 
one  cell  (b)  is  larger  and  is  preparing  to  divide.  //,  later  stage  of  this  division.  /,  stage  of  three  seg- 
mentation spheres  (a  and  c,  c)  resulting  from  this  division. 

cle.  The  entoderm  gradually  expands  until  it  forms  a  complete  second 
layer  within  and  concentric  with  the  outer  stratum  of  the  blastodermic  wall. 
Meanwhile  a  third  layer  of  cells,  the  mesode7'm,  makes  its  appearance  be- 
tween the  ectoderm  and  the  entoderm  and,  in  time,  converts  the  wall  of  the 
blastodermic  vesicle  into  a  trilaminar  envelope.  The  three  cell-sheets 
derived  from  the  inner  cell-mass  constitute  the  blastodermic  or  germ-layers 
— structures  of  great  importance,  since  they  supply  the  cells  from  which  all 
parts  of  the  embryo  are  developed.  The  histological  characters  of  the  outer 
and  inner  of  these  primary  layers  differ,  almost  from  the  first,  from  those  of 
the  mesoderm,  their  component  elements  being  more  compact  in  arrange- 
ment and  early  acquiring  the  characteristics  of  covering  cells  or  epithelium. 


14 


NORMAL    HISTOLOGY. 


The  mesodermic  elements,  on  the  contrary,  for  the  most  part  assume 
irregular  forms  and  are  loosel)!^  held  together  by  intercellular  substance,  thus 


Outer  cells 


Zona  pellucida 


Inner  cells 


Trophoblast 


Ectoderm 
Entoderm 


Trophoblast 
Zona  pellucida 

Fig.  II. — Diagrams  of  very  early  stages  of  the  mammalian  blastodermic  vesicle;  ^4,  the  vesicle  con- 
sists of  trophoblast  and  inner  cell-mass  ;  B,  the  inner  cell-mass  is  differentiating  into  ectoderm  and  ento- 
derm.    [After  van  Beneden.) 

foreshadowing  the  features  which  distinguish  many  of  their  derivatives  as 
members  of  the  connective  tissue  group. 

B 


Fig.  12. — Diagrams  of  later  stages  of  the  mammalian  blastodermic  vesicle  ;  A,  the  thickened  embryonic 
ectoderm  corresponds  to  the  area  in  which  the  embryo  will  develop ;  B,  the  mesoderm  is  ap))earing  as  the 
third  germ-layer  between  the  ectoderm  and  the  entoderm,  the  latter  now  forming  a  complete  layer. 

The  mesoderm  undergoes  important  modifications,   splitting  into  two 
sheets,  ■a  parietal  and  a  visceral  layer,  between  which  is  included  the  primitive 


Ectoderm      Neural  groove 


Medullary  fold 


Fig.  13. — Transverse  section  of  rabbit  embryo  of  about  eight  and  one-half  days,  showing  the  character  of 
the  early  germ-layers;  the  future  neural  canal  is  represented  by  the  widely  open  groove.     X  80. 

body-cavity  or  ccelom.      Subsequently  this  space  is  subdivided  into  the  great 
serous  sacs  of  the  body — the  pericardial,  the  pleural  and  the  peritoneal — 


THE   GERM-LAYERS.  15 

lined  with  the  modified  mesodermic  elements,  known  as  mesothelium  (page 
21 ).  The  cleavage  of  the  middle  germ-layer,  however,  does  not  involve  the 
mesoderm  in  the  immediate  vicinity  of  the  embryonic  axis,  since  on  each 
side  of  the  latter  there  remains  a  tract  of  wwcX^ix. paraxial  mesoderm,  in  which 
appears  a  series  of  quadrilateral  areas,  the  mesodermic  somites.  These  are 
important  since  they  contribute  the  material  giving  rise  to  the  vertebral 
column  and  the  voluntary  muscles. 

The  parietal  layer  of  the  mesoderm  adheres  to  the  ectoderm  and,  in 
conjunction  with  the  latter,  constitutes  the  somatopleural  the  ecto-meso- 
dermic  sheet  that  forms  the  ventro-lateral  walls  of  the  body.  In  like  man- 
ner, the  visceral  laver  adheres  to  the  entoderm  and,  with  it,  constitutes  the 
splanchnopleura,  whose  folding-off  establishes  the  digestive  tube. 

Since  these  primary  layers  give  rise  to  all  the  tissues  of  the  body,  a 
svnopsis  of  their  genetic  relations  may  be  given;  a  word  of  caution,  how- 
ever, should  be  added  against  regarding  these  groups  as  too  sharply  de- 
fined, since  a  certain  degree  of  transition  must  be  recognized. 

DERI\'ATIVES   OF   THE   BLASTODERMIC   LAYERS. 
From  the  ectoderm  are  derived : — 

Epithelium  of  outer  surface  of  the  body,  including  that  of  the  conjunctiva 
and  anterior  surface  of  the  cornea,  and  of  the  external  auditory 
canal,  together  with  the  epithelial  appendages  of  the  skin,  as  hairs, 
nails,  sebaceous-  and  sweat-glands  (_including  the  involuntary  muscle 
of  the  latter). 

Epithelium  of  the  nasal  fossa,  with  its  glands,  as  well  as  the  cavities 
connected  therewith. 

Epithelium  of  the  mouth  and  of  the  salivary  and  other  glands  opening 
into  the  oral  cavity. 

Enamel  of  the  teeth. 

Tissues  of  the  nervous  system. 

The  retina  ;  the  crystalline  lens  and  perhaps  part  of  the  vitreous  humor 
and  of  the  muscle  of  the  iris. 

Epithelium  lining  the  membranous  labyrinth. 

Epithelium  of  the  pituitary  and  pineal  bodies. 

From  the  mesoderm  are  derived : — 

Connective  tissues,  including  areolar  tissue,  tendon,  cartilage,  bone  and 
dentine. 

Muscular  tissue,  except  that  of  the  sweat-glands  and  the  dilator  pupillae. 

Tissues  of  the  vascular  and  lymphatic  systems,  including  their  endo- 
thelium and  circulating  cells. 

All  parts  of  the  sexual  glands  and  their  excretory  passages,  as  far  as  the 
termination  of  the  ejaculatory  ducts  and  of  the  vagina. 

All  parts  of  the  kidney  and  ureter. 

From  the  entodervi  are  derived : — 

Epithelium  of  the  digestive  tract,  with  that  of  all  glandular  appendages, 
except  those  portions  of  ectodermic  origin  at  the  beginning  (oral 
cavity)  and  termination  of  the  tube. 

Epithelium  of  the  respiratory  tract. 

Epithelium  of  the  urinary  bladder  and  of  urethra  (except  part  01  male). 

Epithelium  of  thyroid,  parathyroid  and  thymus  bodies. 


i6  NORMAL   HISTOLOGY. 


THE  ELEMENTARY  TISSUES. 

The  various  parts  and  organs  of  the  complex  body  may  be  resolved,  in 
their  structure,  into  four  groups  of  ele7nentary  tissues — the  epithelial,  the 
connective,  the  mtiscular  and  the  nervous.  By  the  association  and  modifica- 
tion of  two  or  more  of  these  tissues  the  organs  are  made  up  and  acquire  the 
distinctive  characteristics  demanded  by  their  function.  A  fifth  group — the 
vascular  tissues,  including  the  blood-vessels  and  lymphatics  with  the  con- 
tained blood  and  lymph — is  sometimes  added  in  view  of  the  usual  occurrence 
of  these  tissues  as  constituents  of  organs.  Since,  however,  the  vascular  tis- 
sues are  genetically  related  closely  with  the  connective  tissues  and,  where 
highly  specialized,  are  themselves  composite  in  structure,  it  seems  more 
appropriate  that  they  be  not  regarded  as  an  independent  group. 

THE   EPITHELIAL  TISSUES. 

The  epithelial  tissues  include,  primarily,  the  sheet  of  protecting  cells 
(epidermis)  covering  the  exterior  of  the  body  and  the  epithelium  lining  the 
digestive  tube.  Secondarily,  they  constitute  the  derivations  of  the  epider- 
mis, as  hairs,  nails  and  glands  of  the  skin,  and  the  lining  of  the  ducts  and 
compartments  of  the  glands  connected  with  the  digestive  canal,  as  well  as 
the  lining  of  the  respiratory  tract,  which  originates  as  an  evagination  from 
the  gut-tube.  Further,  epithelium  forms  the  lining  of  the  genito-urinary 
tract. 

The  primary  purpose  of  the  epithelium  being  to  protect  the  delicate  vas- 
cular and  nervous  structures  lying  within  the  subjacent  connective  tissue  of 
the  skin  and  of  the  mucous  membranes,  the  epithelial  cells  are  arranged  as  a 
continuous  sheet,  the  individual  elements  being  united  by  a  very  small  amount 
of  intercellular  or  cement  substance. 

Epithelium  is  devoid  of  blood-vessels,  the  necessary  nutrition  of  the 
tissue  being  maintained  by  the  absorption  of  nutritive  juices  which  pass  to 
the  cells  by  way  of  the  minute  clefts  within  the  intercellular  substance.  The 
distribution  of  nerve-fibres  within  epithelium  ordinarily  is  scanty,  although 
in  localities  possessing  a  high  degree  of  sensibility,  as  the  tactile  surfaces  or 
the  cornea,  the  terminal  nerve-filaments  may  lie  between  the  epithelial  ele- 
ments. Frequently  the  epithelium  is  separated  from  the  connective  tissue 
upon  which  it  rests  by  a  delicate  basement  membrane  or  inembrana  propida. 
The  latter  usually  appears  as  a  linear  subepithelial  boundary,  being  often 
particularly  well  marked  beneath  the  epithelium  of  glands. 

Based  on  the  predominating  form  of  the  component  cells,  the  epithelial 
tissues  are  divided  into  two  chief  groups,  squamous  and  cohimnar,  each  of 
which  is  subdivided  into  simple  and  stratified,  according  to  the  presence  of 
a  single  or  several  layers  of  cells  respectively.  Modified  epithelium  includes 
cells  which  exhibit  adaptation  and  specialization  to  meet  particular  uses; 
such  are  the  ciliated,  pigmented  and  glandular  epithelia.  Highly  differenti- 
ated yieuro-epithelium  occurs  in  the  perceptive  apparatus  concerned  in  the 
special  senses,  the  gustatory  cells  of  the  taste-buds  found  on  the  tongue,  the 
rod-  and  cone-cells  of  the  retina  and  the  auditory  cells  of  Corti's  organ  being 
familiar  examples. 

Squamous  Epithelium. — Where  this  variety  of  epithelial  tissues 
occurs  as  a  single  layer,  it  consists  of  flattened  polyhedral  nucleated  plates 
which,  viewed  from  the  surface,  form  a  more  or  less  regular  mosaic.     Hence 


THE   EPITHELIAL   TISSUES. 


17 


the  terms  ' '  pavement  "  or  "  tessellated  ' '  are  sometimes  applied  to  this  type 
of  epithelium.  Such  arrangement  of  the  squamous  type,  however,  is  unus- 
ual in  the  human  body — the  alveoli  of  the  lungs,  the  posterior  surface  of  the 
anterior  capsule  of  the  crystalline  lens  and  the  membranous  labyrinth  being 
the  chief  localities  in  which  simple  squamous  epithelium  is  found. 

The  far  more  usual  disposition  of  the  squamous  type  of  epithelium  is  as 
a  number  of  superimposed  layers,  this  constituting  the  important  group  of 
stratified  squamous  epithelium.      Although  the  free  surface  of  such  struct- 


^>  _^- 


r\ 


|v- 


•V 


Fig.  14. — Simple  squamous  epi- 
thelium from  the  anterior  capsule 
of  the  crystalline  lens.     X  360. 


.®|¥|@(  i^[®^* ©g^!®,: 

Fig.  15. — Stratified  squamous  epithelium  from  anterior  surface 
of  the  cornea.     X  465. 


ures  presents  the  mosaic  formed  by  the  superficial  plates,  the  entire  tissue  is 
by  no  means  composed  of  flattened  cells.  When  seen  in  section  (Fig.  15), 
the  deepest  cells  are  not  scaly,  but  irregularly  columnar,  resting  on  the  base- 
ment membrane  by  slightly  expanded  bases.  The  surface  of  the  underlving 
connecti\e  tissue  is  beset  with  minute  elevations  or  papillae,  which  serve  as 
advantageous  positions  for  the  terminations  of  blood-vessels  or  of  nerves. 
Owing  to  the  more  favored  nutrition  of  the  deepest  stratum,  the  cells  next 
the  connective  tissue  possess  the  greatest  vitality  and  are  the  source  of  the 


cqpcv 


^ 


Fig.  16. — Isolated  surface  cells  from  epithelium 
liniiig  ihe  mouth.     X  320. 


Fig.  17. — Epithelial  cells  from  epidermis,  show- 
ing intercellular  bridges.     X  675. 


new  elements  necessary  to  replace  the  old  and  effete  cells  which  are  continu- 
ally being  removed  at  the  free  surface.  This  loss  is  due  not  only  to  mechan- 
ical abrasion,  but  also  to  the  displacement  of  the  superficial  elements  by  the 
new  cells  formed  within  the  deeper  layers. 

Passing  from  the  basement  membrane  towards  the  free  surface,  the 
form  of  the  cells  undergoes  a  radical  change.  The  columnar  type  belongs 
exclusively  to  the  deepest  layer;  the  superimposed  cells  assume  irregularly 
polyhedral  forms  and  gradually  expand  parallel  to  the  free  surface,  to 
become,  finally,  the  large  thin  scales  (Fig.  i6j  so  characteristic  of  the  super- 


i8  NORMAL    HISTOLOGY. 

ficial  layers  of  stratified  squamous  epithelium.  The  position  of  the  nucleus 
also  varies  with  the  situation  of  the  cells,  since  within  those  next  the  base- 
ment membrane  the  relatively  large  nucleus  lies  near  the  subjacent  con- 
nective tissue,  while  within  the  cells  of  the  middle  and  superficial  strata  the 
nucleus,  comparatively  small,  is  placed  about  the  centre  of  the  cell.  The 
irregularly  polyhedral  cells  of  the  deeper  strata  frequently  are  connected  by 
delicate  protoplasmic  processes  which  bridge  the  intervening  intercellular 
clefts  (Fig.  17);  when  such  elements  are  isolated,  the  delicate  connecting 


Fig.  18. — Transitional  epithelium  from  the  bladder. 
X  300- 


Fig.  19. — Simple  columnar  epithelium  from  ir 
testinal  mucosa.     X  750. 


threads  are  broken  and  appear  as  minute  spines  besetting  the  so-called /»rzV/^/^ 
cells.  In  certain  localities,  as  in  the  urinary  bladder,  the  columnar  cells  of 
the  deepest  layer  rapidly  assume  the  scaly  character  of  the  superficial  elements. 
Such  epithelium  possesses  relatively  few  layers  and  is  often  described  as 
transitional  epitheliiim,  a  modification  of  the  stratified  squamous  variety. 

Columnar  Epithelium. — When  consisting  of  a  single  layer  of  pris- 
moidal  elements,  the  epithelium  constitutes  the  simple  columnar  variety, 
which  is  much  more  widely  distributed  than  the  corresponding  squamous 
group,  the  lining  of  the  stomach  and  of  the  intestinal  tube  being  important 
examples.  When  the  single  layer  of  cells  is  replaced  by  several,  as  in  the 
stratified  columnar  epithelium,  the  superficial  elements  alone  are  typically 
columnar.  The  free  ends  of  the  prismoidal  cells  frequently  present  cyto- 
plastic  specializations  in  the  form  of  a  cuticular  border  or  of  cilia,  while  their 
ends  which  rest  upon  the  basement  membrane  are  pointed,  club-shaped,  or 
forked.  The  intervals  formed  by  such  irregular  contours  are  occupied  by 
the  smaller    cells    of    the  deeper    stratum.      Each  cell   is    provided  with  a 


Fig.  20. — Stratified  columnar  epithelium  from 
vas  dele! ens.     X  500. 


Fig.   21. — Goblet-cells   from   epithelium   lining 
large  intestine,     a  500. 


nucleus,  which  is  situated  about  the  middle  within  the  superficial  elements 
and  nearer  the  base  within  the  deeper  ones.  The  surface  cells  often  contain 
collections  of  mucous  secretion  and,  in  consequence,  become  distended  into 
conspicuous  chalice  forms  known  as  goblet-cells.  Such  modified  elements 
occur  in  great  profusion  within  the  epithelial  lining  of  the  large  intestine 
and  of  the  repository  tract. 

Modified  Epithelium. — In  order  to  meet  particular  work,  beyond 
the  mere  function  of  protection,  epithelial  cells  may  undergo  profound  mod- 
ification or  high  specialization.      Thus,  in  order  to  produce  a  current  favor- 


THE   EPITHELIAL   TISSUES.  19 

able  for  the  propulsion  of  mucus  or  secretions,  the  free  surface  of  the  epi- 
thelium in  many  localities,  as  in  the  trachea  and  bronchial  tubes,  the  inferior 
and  middle  nasal  meatuses  and  the  oviducts  and  uterus,  is  provided  with 
minute  hair-like  vibratile  processes,  or  cilia.  The  exact  relations  of  the  cilia, 
specializations  of  the  substance  of  the  cell,  to  the  cytoplasm  are  still  uncertain, 
although  it  is  probable  that  the  hair-like  processes  attached  to  the  free 
surface  are  connected  with  delicate  intracellular  librillse  within  the  superficial 
and  more  highly  specialized  parts  of  the  cells.  In  man  and  the  higher 
mammals  ciliated  epithelium  is  limited  to  the  columnar  variety.  The 
number  of  cilia  attached  to  each  cell  varies,  but  there  are  usually  between 
one  and  two  dozen  such  appendages.  Their  length,  likewise,  difiers,  those 
lining  the  epididymis  being  about  ten  times  longer  than  those  attached  to 
the  tracheal  mucous  membrane.  L'nder  favorable  conditions,  including  a 
surticient  supply  of  moisture,  oxygen  and  heat,  ciliary  motion  may  continue 
for  many  hours  or  e\'en  days  after  removal  of  the  tissue. 

The  cytoplasm  of  epithelial  cells  often  is  in\-aded  by  particles  of  foreign 
substances;  thus,  granules  of  fatty  and  proteid  matters  are  very  common, 
while  the  presence  of  granules  of  keratohyalin  in  certain  cells  of  the  epidermis 


,  Ciliated 

!^'--^H  '■z^^  Dorder 


cd?? 


Fig.  25. — Ciliated  epithelial  cells.     A, 
. — Straliried    ciliated   columnar   epithelium  from  intestine  of  a  nioUusk  ;  y?.  from  nasal 

fr6m  trachea.     X  500.  mucosa  of  frog.    X  675.    (Eugelmanu.) 

characterizes  the  stratum  granulosum.  When  the  contained  particles  are 
colored,  as  when  composed  of  melanin,  the  affected  cells  acquire  a  dark 
brown  tint  and  are  then  known  as  pigmented  epithelium.  Examples  of  such 
cells  are  seen  in  the  outer  layer  of  the  retina  and  in  deep  cells  of  the  epider- 
mis in  certain  races. 

On  surfaces  clothed  with  columnar  epithelium,  manv  cells  are  distin- 
guished by  unusually  clear  cytoplasm  and  exceptional  form  and  size.  These 
are  the  goblet-ceils,  whose  peculiar  chalice  form  results  from  an  accumulation 
of  mucoid  secretion  elaborated  within  the  cytoplasm  of  the  cells.  When  dis- 
tention becomes  too  great,  the  cell  ruptures  in  the  direction  of  least  resist- 
ance and  the  secretion  is  poured  out  upon  the  surface  of  the  mucous  mem- 
brane as  the  lubricating  mucus.  The  goblet-cells,  therefore,  may  be 
regarded  as  unicellular  glands  and  as  representing  the  simplest  phase  in  the 
temporary  specialization  of  glandular  tissue.  When  the  epithelial  elements 
become  permanently  modified  to  engage  in  the  elaboration  of  secretions, 
they  are  recognized  3.s  glandular  epithelium.  The  cells  lining  the  ducts  and 
the  ultimate  compartments  of  the  glands  are  modified  extensions  of  the  epi- 
thelium covering  the  adjoining  mucous  membrane.  Their  form  and  condi- 
tion depend  upon  the  degree  of  specialization,  varying  from  columnar  to 


20 


NORMAL    HISTOLOGY. 


Fig.  24. — Pigmented  epi- 
thelium from  the  human 
retina.     X  435- 


spherical  and  polyhedral,  in  the  one  case,  and  upon  the  number  and  nature 
of  the  secretion  particles  in  the  other.  The  cells  lining  parts  of  certain 
glands,  as  those  clothing  the  ducts  of  the  salivary  glands,  or  the  convoluted 
portion  of  the  uriniferous  tubules,  exhibit  a  more  or 
less  evident  striation.  Such  cells  constitute  rod  epi- 
thelium. 

The  highest  and    most  complex  modifications  of 
epithelial  tissues  are  those  occurring  during  the  de\'el- 
opment  of  the  structures  designed  to  receive  the  stimuli 
giving  rise  to  the  special  senses.     The  epithelium  in 
these   localities    is  differentiated    into   tv.o    groups  of 
elements,  the  sustentacular  and  the  perceptive;  to  the 
latter  the  name  of  neuro-epitheliuvi  is  applied.     Con- 
spicuous examples  of  such  specialized  epithelium  are 
the  rod-  and  cone-cells  of  the  retina  and  the  auditory 
or  hair-cells  of  Corti's  organ  in  the  internal  ear. 
A  more  detailed  description  of  glandular  epithelium  is  given  in  the 
chapter  devoted  to  Mucous  Membranes  and  Glands  (page  119);  the  details 
of  the  neuro-epithelial  structures  are  included  under  the  appropriate  Organs 
of  Sense. 

THE  ENDOTHELIA. 

This  term,  as  here  used,  is  applied  to  the  modified  mesodermic  cells 
that  cover  serous  surfaces  and,  therefore,  includes  the  lining  of  the  pericardial, 
pleural  and  peritoneal  subdivisions  of  the  body-cavity,  together  with  the 
lining  of  the  blood-  and  lymph-vessels  and  of  the  lymphatic  spaces  through- 
out the  body.  In  principle,  these  spaces  are  intramesodermic  clefts  and  the 
elements  forming  their  lining  are  derivatives  of  the  great  connective  tissue- 
producing  germ-layer.  The  endothelia,  therefore,  are  closely  related  to  the 
connective  tissues  and,  in  a  sense,  may  be  regarded  as  modified  elements  of 
that  class.  In  view  of  their  arrangement  as  investing  cell-sheets  and  other 
resemblances,  they  may  be 
conveniently  discussed  in  con- 
nection with  the  epithelial 
tissues;  indeed  by  many  his- 
tologists  they  are  included 
among  the  epithelia. 

The  most  striking  differ- 
ence in  situation  between  the 
endothelia  and  the  epithelia  is 
the  fact  of  the  former  covering 
surfaces  not  communicating 
with  the  atmosphere,  w^hile  the 
epithelial  tissues  clothe  mu- 
cous membranes,  all  of  which 
are  directly  or  indirectly  con- 
tinuous with  the  integumen- 
tary surface.  A  further  con- 
trast between  these  tissues  is 
presented    .in     their    genetic 

relations  with  the  germ-layers,  since  the  epithelia,  with  the  exception  of  those 
lining  certain  parts  of  the  genito-urinary  tracts  which  are  deriA-ed  from  the 
mesoderm,  are  transformations  and  outgrowths  from  the  ectoderm  and  ento- 


FiG.  25. — Mesothelial  cells  from  surface  of  omentum;  intercel- 
lular cement-substance  stained  by  argentic  nitrate.    X  3°°- 


CONNECTIVE   TISSUE. 


21 


derm,  while  the  endotheUa  iire  direct  modifications  of  mesodermic  cells.  The 
young  mesodermic  cells  bordering-  the  early  body-cavity  become  differentiated 
into  a  delicate  lining  for  this  sjxice,  and  later  gi\'e  rise  to  the  plate-like  ele- 
ments which  constitute  the  lining  of  the  permanent  serous  sacs.  The  primary 
lining  is  known  as  the  iiwsot/uiiiim,  which  name  is  often  retained  to  designate 
the  investment  of  the  pericardial,  pleural  and  peritoneal  cavities,  as  distin- 
guished from  the  endothelium  which  lines  the  vascular  and  other  serous  spaces. 

As  seen  in  typical  preparations,  obtained  from  the  peritoneum  after 
appropriate  treatment  with  argentic  nitrate,  the  endothelial  cells  on  surface 
\'iew  appear  as  irregularly  polygonal  areas  mapped  out  by  deeply  tinted 
lines  (Fig.  25).  The  latter  represent  the  silver-stained  albuminous  inter- 
cellular cement-substance,  which  joins  the  flattened  cells  in  a  manner  similar 
to  that  seen  in  simple  squamous  epithelium.  The  lines  of  apposition  are 
sinuous  and  less  regular  than  those  between  epithelial  cells,  in  many  cases 
the  lines  appearing  distinctly  serrated.  The  form  of  the  cells  and  the  char- 
acter of  their  contours,  however,  are  not 
constant,  since  they  are  greatly  influenced 
by  the  degree  of  tension  to  which  the 
tissue  has  been  subjected.  Protoplasmic 
threads  directly  connecting  the  adjoining 
cells  have  been  described. 

After  silver  staining  the  intercellular 
substance  frequently  shows  irregular, 
deeply  colored  areas  at  points  where 
several  endothelial  cells  come  together. 
These  figures  are  described  as  stigmata 
or  pseudostomata  and  by  some  are  inter- 
preted as  indications  of  the  existence  of 
.  minute  openings  leading  from  the  serous 
cavity  into  the  subjacent  lymphatics.  They  are,  however,  largely  accidental 
and  due  to  dense  local  accumulations  of  the  stained  intercellular  materials. 
True  orifices,  or  stomata,  on  the  other  hand,  undoubtedly  exist  in  certain 
serous  membranes,  as  in  the  septum  between  the  peritoneal  cavity  and  the 
abdominal  lymph-sac  of  the  frog  and,  probably,  the  peritoneal  surface  of  the 
diaphragm  of  mammals.  The  positions  of  these  stomata  are  marked  by  a 
conspicuous  modification  in  the  form  and  arrangement  of  the  surrounding 
endothelial  plates,  which  are  radially  disposed  about  centres  occupied  by  the 
stomata.  The  immediate  walls  of  the  orifices  are  formed  by  smaller  and 
more  granvilar  elements,  the  guard-cells,  the  contraction  and  expansion  of 
which  probably  modify  the  size  of  the  openings. 


Fig.  26. — Endothelial  cells  lining  artery  of 
dog;   stained   with  silver   and    hematoxylin. 

X  450- 


THE   CONNECTIVE  TISSUES. 

The  important  group  of  connective  substances,  the  most  widely  distrib- 
uted of  all  tissues,  is  the  direct  product  of  the  middle  germ-layer.  Since  the 
latter  is  also  the  origin  of  epithelial,  muscular,  vascular  and  lymphoid  tissues, 
that  portion  of  the  mesoderm  especially  concerned  in  producing  the  connec- 
tive tissues  has  been  designated  the  mesenchyma.  Their  essential  role,  con- 
nection and  support,  being  largely  passive  and  mechanical,  the  physical 
characteristics  of  these  tissues  are  of  much  importance.  These  depend  upon 
the  intercellular  substance,  which,  in  marked  contrast  to  the  meagre  cement- 
substance  of  the  epithelia,  is  very  large  in  amount  and  contributes  the  chief 
bulk  of  the  tissue. 


22 


NORMAL    HISTOLOGY. 


During  the  period  of  embryonal  growth  the  intercellular  substance  is 
semifluid,  gelatinous  and  plastic;  a  little  later,  as  growing  connective  tissue, 
it  is  still  soft,  although  more  definitely  formed;  while,  as  the  adult  areolar 
tissue,  it  becomes  tough  and  yielding.  Grouped  as  masses  in  which  fibrous 
tissue  predominates,  the  intercellular  substance  acquires  the  toughness  and 
inextensibihty  of  tendon;  where,  on  the  contrary,  large  quantities  of  elastic 
tissue  are  present,  as  in  certain  ligaments,  extensibility  is  conspicuous. 
Further  condensation  of  the  intercellular  substance  produces  the  resistance 
encountered  in  hyaline  cartilage,  intermediate  degrees  of  condensation  being 
presented  by  the  fibrous  and  elastic  varieties.  In  those  cases  in  which  the 
ground-substance  becomes  impregnated  with  calcareous  salts,  the  hardness 
of  bone  or  of  dentine  results.  Notwithstanding  these  variations  in  the 
density  and  physical  properties  of  the  intercellular  substance,  the  cellular 

elements  have  undergone  little 
radical  change,  the  connective 
tissue-corpuscle,  the  tendon-cell, 
the  cartilage-cell  and  the  bone- 
corpuscle  being  essentially  iden- 
tical. 

The  principal  forms  in  which 
connective  tissue  occurs  are:  (i) 
Mucous  Tissue,  (2)  Reticular  Tis- 
sue, (3)  Pibrous  Tissue — loose 
and  dense,  (4)  Adipose  Tissue, 
(5)  Cartilage  and  (6)  Bone. 

Mucous  Tissue. — This 
form  of  connective  substance  is 
the  most  immature,  being  in  fact 
the  embryonal  type,  and  closely 
resembles  the  parent  tissue,  the 
mesenchyma.  As  seen  in  sec- 
tions of  the  embryo  or  of  the  early  umbilical  cord,  it  consists  of  a  delicate 
protoplasmic  network  containing  a  semifluid  intercellular  substance.  The 
network  is  formed  by  the  union  of  the  processes  of  irregularly  branched 
stellate  or  fusiform  cells,  whose  oval  nuclei  are  embedded  in  plate-like 
masses  of  faintly  granular  cytoplasm.  The  intercellular  ground-substance 
is  semifluid  and,  depending  upon  the  stage  of  development,  either  struct- 
ureless or  traversed  by  indistinct  fibrillae.  The  latter  owe  their  origin  to 
the  cells  and  are  produced  by  differentiation  of  the  cytoplasm.  Being 
essentially  embryonal  tissue,  in  the  higher  animals  the  mucous  variety 
of  connective  tissue  is  limited  to  the  earlier  stages  of  development,  the 
so-called  jelly  of  Wharton  in  the  young  embryo  being  a  striking  example. 
Among  the  invertebrates,  on  the  other  hand,  mucous  tissue  is  formed  in 
the  adult  animal.  Certain  pathological  growths,  known  as  myxoma,  exhibit 
a  similar  arrangement  of  cells  and  yield  mucus.  The  latter  substance, 
produced  also  by  glandular  epithelium,  contains  true  mucins — a  group  of 
complex  proteid  substances. 

Reticular  Tissue. — This  variety  of  connective  tissue  differs  from  the 
mucous  in  retaining  only  a  very  meagre  amount  of  intercellular  substance 
and  consists,  therefore,  chiefly — in  some  instances  almost  exclusively — of  a 
network  of  connective  tissue  cells,  the  meshes  of  which  are  occupied  by  fluid 
and  the  lymphoid  elements  which  the  reticulum  supports.  The  cells  are 
flat  and  stellate  and  rest  upon  the  surface  of  the  strands  of  intercellular  sub- 


FiG.   27.- 


■Mucous   tissue   from 
umbilical  cord. 


section   of   very  young 

X  35°- 


CONNECTIVE   TISSUE. 


23 


-^ 


Fig.  28. — Reticular  tissue  from  lymph- 
node.     X  300. 


Stance.  Where  the  latter  is  best  developed,  it  is  composed  of  delicate  fibres 
which  resemble  those  of  fibrous  connective  tissue.  The  reticular  fibres  prob- 
at)ly  differ  somewhat  from  the  white  fibres  of  connective  tissue  in  chemical 
composition,  containing  a  variety  of  gelatin  known  as  reticnlin.  Reticular 
tissue  occurs  principally  as  the  supporting 
framework  of  lymphoid  tissue,  hence  is  well 
seen  in  suitably  prepared  sections  of  the 
lymph-nodes  and  of  the  spleen.  It  is  also 
found  in  the  mucous  membrane  of  the  intes- 
tinal tract,  while  the  reticulum  of  bone-marrow 
and  the  interstitial  tissue  of  certain  organs, 
as  the  kidneys  and  liver,  contain  it. 

Fibrous  Tissue. — Under  this  head  are 
included  the  more  usual  forms  of  connective 
tissue  which  have  representation  in,  practi- 
cally, all  parts  of  the  body.  They  exhibit 
a  wide  range  of  variation  in  their  physical 
properties  which  depend  upon  differences 
in  the  intercellular  substance,  due  to  modi- 
fications in  the  arrangement  and  propor- 
tions of  its  constituents.  Before  considering  the  several  varieties  of  fibrous 
connective  tissue,  loose  and  dense,  the  histological  components  common 
to  all  these  tissues  claim  attention.  These  coinponents  are  the  cells  and 
\^\&  fibres. 

Connective  Tissue  Cells. — Although  the  more  active  constituent  of  the 
connective  tissue,  it  is  only  in  the  youngest  and  immature  stages  that 
the  cells  are  conspicuous;  later,  after  the  tissue  has  acquired  its  definite 
characteristics,  the  intercellular  substance  has  usually  become  so  predomi- 
nant, that  the  cells  are  reduced  to  inconspicuous  elements,  notwithstanding 
their  important  role  as  nutritive  and  reproductive 
;,.,  centres.     The  irregularly  branched  or  stellate  types 

.iy-,  ,         of    the    parent    mesenchymal    cells    are    retained 

'"'^'       :\  '     ordinarily  only  during  the  earlier  periods  of  growth, 
%        the  connective  tissue  cells  decreasing  in   size  and 
{        prominence  as  the  intercellular  substance  increases 
:,,  in  amount  and  differentiates  into  definite    bundles 

/         _:,     5.         of  fibres.      In  the  adult  tissues,  with  few  exceptions, 
the    cells    appear   as    small    fusiform    or    flattened 
'A      -:  elements,  in  which  the  deeply  staining  oval  nucleus, 

'^-  surrounded  by  a  small  amount  of  cytoplasm,  serves 

:'  as    the    chief    means  of   detection.      Being   thicker 

than  the  cell-body,  the  nucleus  projects  beyond 
the  general  level  of  the  cell  and,  viewed  in  profile, 
appears  as  a  colored  linear  elevation  embedded 
in  a  plate  of  faintly  tinged  cytoplasm.  Since 
the  cells  depend  for  their  nutrition  on  the  tissue- 
juices  which  occupy  the  clefts  or  lymph-spaces 
between  the  bundles  of  fibres,  the  relation  of  the  connective  tissue  cells 
to  these  bundles  is  constant  and  characteristic  for,  wherever  definite 
bundles  are  present,  the  cells  are  applied  to  the  surface  of  the  fasciculi. 
Where  the  latter  are  closely  packed,  the  juice-channels  form  a  system  of 
intercommunicating  spaces  or  canals,  well  seen  in  the  cornea  after  staining 
with  argentic  nitrate,  when  they  appear  as  light,  irregularly  stellate  figures 


■>-&v 


Fig.  2q. — Young  connective 
tissue  cells  from  subcutaneous 
tissue  of  cat  embryo.     X  590. 


24 


NORMAL    HISTOLOGY. 


Fig.  30. — Lymph-spaces  within  dense  connective  tissue, 
from  cornea  of  calf;  the  surrounding  ground-substance  has 
been  stained  with  argentic  nitrate.     X  500. 


(Fig.  30),  in  which  are  lodged  the  connective  tissue  cells,  applied  to  the 
wall  formed  by  the  dense  fibrous  tissue.  In  principle,  the  same  arrangement 
holds  good  for  cartilage  and  bone,  since  in  these  tissues  the  cells  lie  within 
the  lacunae.     The  larger  branched  connective  tissue  cells  sometimes,  as  when 

subjected  to  thermal,  chemical 

or   electrical   stimulus,   exhibit 

changes    in    form,    possessing 

the   power  of  retracting   their 

-^  ,,^ -,^^,  processes  and  displaying  feeble 

'^^         ^!^^^:  amceboid  movement. 

.»%,-v      Wxl^''  In  a  few  localities  in  man — 

the  choroid,  the  iris,  the  sclera, 
the  dermis  and  the  pia  mater 
— but  widely  distributed  in  the 
lower  vertebrates,  the  branched 
connective  tissue  elements  often 
,   .  contain  dark  particles  of  mela- 

■  ?;  nin    and,  therefore,  appear   as 

conspicuous  irregular  figures 
shading  from  brown  to  black. 
Such  elements  are  usually 
spoken  of  2s,  pigment-cells,  be- 
ing, of  course,  only  connective 
tissue  cells  modified  by  the 
invasion  of  the  colored  foreign  material.  Since  this  invasion  is  limited  to 
the  cytoplasm,  the  unaffected  nucleus  appears  as  a  small  light  oval  area  in 
the  midst  of  the  dark  figure  (Fig.  32).  In  the  amphibians  a  favorite  situa- 
tion of  pigmented  cells  is  the  immediate  vicinity  of  blood-vessels,  and  it  is 
probable  that  at  times  the  connective  tissue  cells,  as  well  as  leucocytes,  may 
take  up  colored  particles  derived  from  the  blood.  In  addition  to  the  melanin 
series  and  the  hemoglobin  derivatives,  a  third  group  of  pigments,  the  lipo- 
chromes,  is  derived  from  fat.  A  very  common  modification  of  the  connective 
tissue  element  is  the  appearance  of 
droplets  of  oil  within  its  cytoplasm. 
When  such  invasion  becomes  ex- 
tensive, the  element  becomes  a 
fat-cell  and  a  constituent  of  adi- 
pose tissue.  Further  consideration 
of  the  fat-cells  will  be  deferred 
until  adipose  tissue  is  described 
(page  29). 

In  addition  to  the  character- 
istic connective  tissue  cells  and 
their  modifications  containing  pig- 
ment and  fat,  a  variable  number  of 
free  cells  are  encountered  in  the 
less  dense  forms  of  fibrous  tissue. 
Much  uncertainty  exists  as  to  the 
nature  and  source  of  some  of  these 
elements,  and  consequently  it  is  impossible  to  state  definitely  their  relations. 
The  most  constant  of  these  free  cells  are  the  migratoiy  lymphocytes,  which 
escape  from  the  blood-vessels  into  the  interfascicular  clefts.  Being  unat- 
tached to  the  fibres,  they  change  their  position  within  the  tissue  and,  hence, 


Fig.  31. — Connective  tissue  eels,  from  cornea  of  calf, 
which  occupy  spaces  similar  to  those  shown  in  preced- 
ing figure.     X  525- 


CONNECTIVE   TISSUE.  25 

are  often  designated  "wandering  cells"  as  distinguished  from  "  fixed  cells," 
as  the  connective  tissue  elements  proper  are  then  termed.  The  lymphocytes 
exhibit  the  usual  characteristics  of  their  class  (see  Blood,  page  97)  and 
appear  as  small  irregular  cells  in  which  the  spherical  deeply  stained  nucleus 
is  surrounded  by  a  narrow  zone  of  cytoplasm.  Occasionally  larger  elements, 
the  plasiiia-cells,  are  seen,  especially  in  the  vicinity  of  the  blood-channels. 
They  are  probably  derived  from  the  lymphocytes,  but  differ  from  these  in 
their  much  larger  size,  greater  amount  of  readily  staining  cytoplasm,  and 
markedly  eccentric  nucleus.  Their  cytoplasm  stains  deeply  with  basic  dyes 
and  contains  but  few  and  indistinct  granules.  Two  other  forms  of  free 
cells,  the  Diast-cells  and  the  eosinopkiles,  are  conspicuous  by  reason  of  the 
coarse  granules  with  which  their  cytoplasm  is  laden.  The  mast-cells  are 
irregularly  round  or  oval  in  shape  and  possess  an  oval  nucleus  (Fig.  33). 
The    coarse   granules  are.  deeply 

colored   by  basic  dyes  but  prone  a  -' 

to  change.      The  eosinopkiles   are 


Fig.  32. — Pigmented  connective  tissue  cells  from  Fig.  33. — Mast-cells  from  submucous  tissue  of 

choroid.     X  400.  mouth;  v^  v,  blood-vessels.     X  825. 

distinguished  by  large  granules  which,  while  staining  with  acid  dyes,  possess 
an  especial  affinity  for  eosin,  after  the  action  of  which  they  appear  copper-red. 
Under  the  name,  clasmatocytes,  ha\'e  been  described  irregular  branched  cells 
with  long  processes  and  scattered  coarse  granules.  These  elements  are, 
perhaps,  modifications  of  the  mast-cells,  which  they  resemble  in  granulation 
and  staining  reactions. 

Connective  Tissue  Fibres. — The  intercellular  substance  of  fibrous  con- 
nective tissue  includes  two  varieties  of  fibrillar  constituents,  the  ivhite 
fibres  and  the  elastic  fibres.  Both  of  these  probably  arise  by  the  differenti- 
ation of  the  more  peripheral  part  (exoplasm)  of  the  cell-body  of  the  young 
connective  tissue  cells,  which  in  the  earliest  stages  are  united  in  a  common 
cytoplastic  reticulum  or  syncytium.  To  the  agency  of  the  cells,  then,  must 
be  ascribed  the  production  of  the  fibrous  intercellular  substance. 

The  white  fibres  are  grouped  in  more  or  less  definite  bundles,  which, 
as  seen  in  the  usual  teased  preparations  of  areolar  tissue,  exhibit  a  wavy 
longitudinal  striation.  This  marking  is  due  to  the  apposition  of  the  individ- 
ual fibrillae,  which  are  so  thin  as  to  have  no  appreciable  width.  In  the 
denser  forms  of  fibrous  tissue,  as  in  tendon,  the  white  fibres  are  assembled 
in  robust  fasciculi  with  great  regularity  and  so  closely  packed  and  luted 
together  by  cement-substance  that  all  trace  of  the  individual  fibrillae  is  lost, 
the  bundle  appearing  homogeneous,  unless  some  means  is  taken  to  dissoci- 
ate its  component  fibrils.  White  fibres  yield  gelatin  on  boiling  with  water, 
and  consist  chemically  of  an  albuminoid  substance  termed  collagen.  They 
are  not  digested  by  pancreatin  and  on  the  addition  of  acetic  acid  become 
swollen  and  transparent  and,  finally,  invisible. 


26 


NORMAL    HISTOLOGY. 


The  elastic  fibres  usually  occur  as  networks  of  highly  refracting  homo- 
geneous fibrils  which  lie  among  the  bundles  of  white  fibres.  The  individual 
fibres  are  much  thicker  than  the  white  ones  and,  although  differing  in  width, 
maintain  a  constant  diameter  until  augmented  by  fusion  with  other  elastic 


Connective  tissue  cell 


Connective  tissue  cells  ( 


Migratory  colorless 

^•^  blood-cells 


^^     ^  -^ 


Bundles  of  white  fibres 


Elastic  fibres - 


Fig.  34. — Section  of  subcutaneous  tissue,  showing  constituents  of  areolar  tissue.     X  300. 


fibres.  So  long  as  the  tissue  in  which  they  lie  maintains  its  normal  tension, 
the  elastic  fibres  remain  taut  and  approximately  straight,  but  when  disso- 
ciated, as  in  teased  preparations,  they  assume  a  characteristic  form  and 
become  wavy,  bowed  or  coiled.  The  proportion  of  elastic  fibres  in  fibrous 
connective  tissue  is,  ordinarily, 
small,  conferring  only  a  moderate 
degree    of   elasticity.      In    certain 

localities,  however,  as  in  the  liga-  .,  ,  .         '^      -^ 

menta  flava  of  man  and  the  nuchal 


Elastic  fibre 
in  section 


Interfibril- 
lar  connec- 
tive tissue 
Nucleus  of 
eoTinective 
tissue  cell 


\{;^-f  ^13^  •<-i:^::^.-<'' 


Fig.  35. — Portions  of  isolated  elastic  fibres  from 
ligamentum  nuchae  of  ox.     X  375. 


Fig.  36. — Transverse  section  of  ligamentum  nuchse 
of  ox.    X  45°- 


ligament  of  the  lower  mammals,  almost  the  entire  structure  is  made  up  of 
robust  elastic  fibres,  held  together  by  a  small  amount  of  intervening  white 
fibres.      In  transverse  sections  of  such  ligaments  (Fig.  36),  the  individual 


CONNECTIVE   TISSUE. 


0 


Fig.  37. — Fragment 
of  fenestrated  mem- 
brane from  large  ar- 
tery ;  surface  view. 
X  510. 


elastic  fibres  appear  as  minute  polygonal  areas,  separated  by  the  wliite  fibres 
and  the  associated  connecti\-e  tissue  cells.  Where,  on  the  other  hand,  elas- 
ticity would  be  disad\antageous,  as  in  tendons  and  aponeuroses,  very  few 
elastic  fibres  are  present,  the  dense  fibrous  structures  being  composed 
practically  of  white  fibres  alone.  Within  the  walls  of  the 
large  blood-vessels,  the  broad  ribbon-like  elastic  fibres  are 
fused  into  membranous  tracts,  which  contain  numerous 
openings  of  various  size  (Fig.  37)  and  are,  therefore, 
known  as  fenestrated  membranes.  Elastic  fibres  withstand 
dilute  acids  and  alkalies,  consequently  becoming  more  e\i- 
dent  in  tissue  treated  with  acetic  acid  in  which  the  white 
fibres  disappear.  In  their  chemical  composition  they  differ 
from  the  white  fibres,  yielding  elastin  and  not  gelatin  on 
boiling  and  disappearing  upon  being  subjected  to  pancre- 
atic digestion.  Likewise,  in  their  staining  reactions  elastic 
fibres  differ  from  the  white;  by  taking  advantage  of  the 
affinity  which  the  former  possess  for  certain  dyes,  as  orcein, 
a  much  wider  and  more  generous  distribution  of  elastic 
tissue  has  been  established  than  was  formerly  appreciated. 

Loose  fibrous  or  areolar  tissue  occurs  throughout  the  body  wher- 
ever the  opposed  parts  although  connected  enjoy  considerable  mobility. 
Familiar  examples  are  the  sheets  or  tracts  of  yielding  connective  tissue,  which 
lie  between  the  skin  and  underlying  fascia  or  beneath  mucous  membranes, 
that  unite  the  muscles  and  assist  in  keeping  the  viscera  in  place.  The 
variable  bundles  of  white  fibres  are  loosely  and  irregularly  disposed,  crossing 

in  all  directions  and  enclosing  cor- 
respondingly indefinite  lymphatic 
clefts.  The  elastic  fibres  form  a 
network  of  highly  refracting  threads 
which,  in  sections  and  teased  prepa- 
rations, are  more  or  less  wavy  and 
curled.  The  cellular  constituents  of 
the  tissue  are  relatively  inconspicu- 
ous, but  here  and  there  the  connec- 
tive tissue  cells  are  seen  as  spindle- 
shaped  or  irregular  plate-like  bodies 
applied  to  the  surface  of  the  fibrous 
bundles.  They  are  bathed  by  the 
tissue-juices  that  well  through  the 
interfascicular  spaces  within  which 
clefts  are  also  lodged  the  migratory 
lymphocytes  and  other  forms  of  free 
cells. 

Dense  fibrous  tissue  owes 
its  characteristics  to  the  more  com- 
pact and  orderly  arrangement  of  the 
bundles  of  white  fibres.  Although 
the  individual  fibres  are  no  thicker 
than  in  the  areolar  tissue,  they  are 
grouped  into  larger  bundles  and  held  more  closely  together  by  the  inter- 
fibrillar  cement  or  ground-substance.  The  bundles  are  disposed  with  greater 
regularity,  either  as  closely  packed  parallel  fasciculi,  as  in  ligaments,  tendon 
and  aponeuroses,  or  as  intimately  felted  bands  fo-ming  t.brous  sheets,  as  in 


Fig.  38  — Portion  of  omentum,  showing  fibro-elastic 
tissue  arranged  as  a  fenestrated  membrane  ;  the  nuclei 
belong  to  the  connective  tissue  and  surface  endothelial 
cells.     X  120. 


28 


NORMAL   HISTOLOGY. 


fascite,  the  cornea  and  the  dura  mater.  In  the  dense  connective  tissue  the 
ground-substance  often  contains  a  system  of  definite  interfascicular  lymph- 
spaces,  which,  in  suitably  stained  preparations,  appear  as  irregularly  stellate 
clefts  (Fig.  30)  that  form,  by  union  of  their  ramifications,  a  network  of  chan- 
nels for  the  conveyance  of  the  tissue-juices  throughout  the  dense  structure. 
Where  definite,  as  in  the  cornea  or  central  tendon  of  the  diaphragm,  these 
spaces  are  almost,  if  not  completely,  filled  by  the  stellate  connective  tissue  cells 
which  they  enclose.  A  somewhat  similar,  although  modified  relation,  is  to 
be  noted  in  the  bursae,  tendon- sheaths  and  smaller  joint-cavities,  in  which 
the  free  inner  surface  is  often  clothed  by  an  incomplete  covering  of  branched 
or  plate-like  connective  tissue  cells. 

Tendon,  the  densest  form  of  fibrous  tissue,  consists  essentially  of  par- 
allel bundles  of  white  fibres.  The  individual  fibres,  held  together  by  cement 
substance,  are  assembled  as  comparatively  large  primary  bimdles  which,   in 


Blood-vessel  within  septa 
enclosing  tertiary  bundles 


k 


Cv' 


m: 


Serondary, 
bundle 


/ 


Primary  bundle 


Fig.  3q. — Longitudinal  section 
of  tendon  from  young  subject ; 
the    tendon-cells    are    seen    in 


Spaces  occupied  by  tendon-cells 


Fig.  40. — Transverse  section  of  a  tendon,  showing  the  grouping 
profile  between  the  bundles  of  of  the  tendon-tissue  into  primary,  secondary  and  tertiary  bundles, 

fibrous  tissue.     X  300.  X  80. 

turn,  are  united  by  the  interfascicular  ground-substance  and  grouped  into 
secondary  bundles.  The  latter,  invested  by  a  delicate  sheath  of  areolar 
tissue  and  partially  covered  by  plate-like  connective  tissue  cells,  are  held 
together  by  partitions  or  septa  of  areolar  tissue  which  are  extensions  of  the 
general  connective  tissue  envelope  that  surrounds  the  entire  tendon.  The 
larger  septa  surrounding  the  tertiary  bundles  support  the  meagre  blood- 
vessels and  nerves  and  afford  a  path  by  which  these  gain  the  interior  of 
the  tendon.  The  blood-vessels,  however,  never  penetrate  the  individual 
bundles,  but  are  confined  to  the  areolar  tissue  which  invests  them.  The 
relations  of  the  nerves  to  the  tendon-tissue  are  described  with  the  Nerve- 
Endings  (page  85).  The  primary  bundles  consist  exclusively  of  white 
fibres  and  the  cement-substance,  which  contain  collagen  and  tendo-mucoid 
respectively.  A  few  delicate  elastic  fibres  are  sometimes  distinguishable  in 
the  vicinity  of  the  tendon-cells.  The  latter  are  the  equivalents  of  the  usual 
connective  tissue  cells,  but  so  modified  by  the  disposition  of  the  bundles  to 
which  they  conform  that  they  assume  distinctive  shapes.  The  tendon-cells 
occur  in  rows  within  the  clefts  between  the  primary  bundles,  upon  the  surface 


ADIPOSE  TISSUE. 


29 


of  which  the  thin  plate-Hke  bodies  are  appHed.  Since  each  cell  is  in 
close  contact  with  two  or  three  bundles,  the  cytoplasm  is  moulded  by 
the  bundles  into  wing-like  expansions.  Seen  from  the  surface,  the  tendon- 
cells  appear  as  small  rectangular  elements, 
whose  round  nuclei  are  disposed  in  pairs, 
the  nucleus  of  one  cell  lying  close  to  that 
Tendon-huiidie— f— li —     jj(     1 1^.^  of  its  neighbor.      Viewed  in  longitudinal 

profile,   the    tendon-cells  appear  as    nar- 

Profile  view 


Oblique  viev  — 


Surface  view 


Fig.  41 — Teiulon-buiidles  from  tail  of 
mouse,  showing- (liRerent  views  of  Ihe  ten- 
don-cells.    X  300. 


Fig.  42. — Transverse  section  of  teiKion-bundles  (b); 
the  interfascicular  spaces  (s)  contain  the  tendon-cells 
(a)  applied  to  the  surface  of  the  bundles.     X  300. 


row^  rods,   while  when  seen  in  transverse  section,   they  present  as  stellate 
figures,  the  extended  limbs  of  which  are  the  sections  of    the  wing-plates. 


ADIPOSE   TISSUE. 

The  fatty  material  contained  within  the  body  is  enclosed,  to  a  large 
extent,  within  connective  tissue  cells  in  various  localities.  These  modified 
elements  are  known  sls  /a/-ce//s,  which,  together  with  the  areolar  tissue  con- 
necting the  cells  and  supporting  the  fair  supply  of  blood-\'essels,  constitute 
adipose  tissue. 

The  distribution  of  adipose  tissue  includes  almost  all  parts  of  the  body. 
Among  the  localities  in  which  the  accumulations  of  fat  are  conspicuous,  are 
the  subcutaneous  areolar  tissue,  the  orbits,  the  marrow  of  bones,  the  mesen- 
tery and  the  omentum,  the  subperitoneal  tissue  and  the  subpericardial  tissue 
of  the  heart,  the  areolar  tissue  surrounding  the  kidneys,  and  the  vicinity  of 
the  joints.  On  the  other  hand,  in  a  few  situations,  including  the  subcuta- 
neous areolar  tissue  of  the  eyelids,  the  penis,  the  clitoris  and  labia  minora, 
the  lungs,  except  near  their  roots,  and  the  interior  of  the  cranium,  adipose 
tissue  is  absent,  even  when  developed  to  excess  in  other  parts.  As  ordina- 
rily seen,  adipose  tissue  is  of  a  light  straw  color  and  often  exhibits  a  gran- 
ular texture  due  to  the  groups  of  fat-cells  within  the  supporting  areolar 
tissue. 

Examined  microscopically  in  preparations  from  localities  where  they 
are  not  crowded  and  retain  their  individual  form,  the  fat-cells  appear  as 
large  clear  spherical  sacs  held  together  by  delicate  areolar  tissue.  Unless 
treated  with  some  stain  possessing  an  especial  affinity  for  fat,  as  osmic  acid 
or  Sudan  III,  the  oily  content  of  the  cells  appears  transparent  and  uncol- 
ored  and  seemingly  occupies  the  entire  cell-body.  Critical  examination, 
however,  demonstrates  tlie  presence  of  an  extremely  thin  peripheral  layer  of 
cytoplasm,  which  completely  surrounds  the  huge  oil-drops  and  at  one  place 
presents    a    local    accumulation    enclosing    the    displaced    and    compressed 


30 


NORMAL    HISTOLOGY. 


nucleus.  In  thin  sections  of  adipose  tissue,  by  no  means  every  fat-cell 
exhibits  a  nucleus,  since,  owing  to  the  small  size  of  the  latter  in  comparison 
with  the  diameter  of  the  cell,  many  sections  include  zones  lying  beyond  the 


'  -f<c--' 


r-^         'I  .* 


'-"Vj'- 


>"•  s 


*    ■"«  -~    .  It 


Fig.  43. — Portion  of  omentum,  showing  groups  of  fat-cells  between  the  bundles  of  connective  tissue.     X  iSO- 

nucleus.  During  life  the  fat  within  the  body  is  fluid.  Quite  often  radiating 
clusters  of  slender /af-aysta/s  are  observed  within  the  adipose  cells.  These 
are  margarin  crystals  that  formed  when  the  fat  solidified  after  death. 

Fat-cells  occur  usually  in  groups  supported  and  held  together  by  highly 
vascular  connective  tissue.  In  localities  possessing  considerable  masses  of 
fat,  as  beneath  the  scalp  and  the  skin,  the  cells  are  grouped  into  lobules 
which  appear  as  yellow  granules  to  the  unaided  eye;  in  such  positions  the 

individual  fat-cells  lose  their 
spherical  shape  and  assume  a 
polyhedral  form  as  the  result 
of  the  mutual  pressure  of  the 
closely  packed  vesicles. 

In  connective  tissue  ele- 
ments about  to  become  fat-cells, 
minute  isolated  oil-drops  first 
appear  within  the  cytoplasm 
in  the  vicinity  of  the  nucleus. 
These  droplets  increase  iti  size 
and  number,  coalesce  and  grad- 
ually encroach  upon  the  cyto- 
plar^m  until  the  latter  is  reduced 
to  a  thin,  almost  inappreciable 
envelope,  which  completely  in- 
vests the  now  huge  distending  oil-drops.  The  nucleus,  likewise,  is  displaced 
towards  the  periphery,  where  it  appears  in  profile  as  an  inconspicuous  cres- 
cent embedded  within  the  cytoplasm  (Fig.  44).      When  the  invasion  of  the 


Nucleus 
"of  older 

fat-cell 


Fig.  44. — Young  fat-cells  from  subcutaneous  tissue.     X  500 


CARTILAGE. 


31 


ceil  is  extensive,  the  nucleus  may  contain  minute  oil-globules.  Although 
many  connective  tissue  elements  become  transformed  into  fat-cells  at  later 
periods,  the  earliest  adipose  tissue  in  some  localities,  as  beneath  the  skin 
and  in  the  orbit  and  omentum,  is  developed  from  highly  vascularized  lobular 
groups  of  mesenchymal  cells  (the /a/  organs  of  Toldt)  which  seem  to  be  set 
apart  for  the  production  of  such  tissue.  During  prolonged  fasting  and 
extreme  emaciation  the  fat-cells  may  lose  the  greater  part,  or  even  their 
entire  quota,  of  oily  contents,  which  is  then  often  replaced  by  a  thin  viscid 
cytoplasm  that  distinguishes  the  so-called  serous  fat-cells.  In  other  cases, 
after  the  disappearance  of  the  oil-drops  the  fat-cells  return  to  a  condition 
closely  resembling  that  of  the  ordinary  connective  tissue  cell. 


r.,- 


-&>,  ••?, 


% 


(. 


>^>  ,^ 


^/ 


CARTILAGE. 

Cartilage  includes  a  group  of  supporting  tissues  in  which  the  intercellu- 
lar substance  undergoes  increasing  condensation  until,  as  the  hyaline  variety, 
the  intercellular  matrix  appears  homogeneous,  the  constituent  fibres  being 
so  compact  and  closely  blended  that  the  fibrous  structure  is  ordinarily  no 
longer  appreciable. 

Depending  upon  the 
differences  exhibited  by 
the  intercellular  matrix, 
three  varieties  of  cartilage 
are  recognized,  hyaline, 
fibrous  and  elastic. 

Hyaline  cartilage, 
or  gristle  (Fig.  45),  is 
widely  distributed,  form- 
ing the  articular  surfaces 
of  the  bones,  the  costal 
cartilages,  the  larger  carti- 
lages of  the  larynx  and 
the  cartilaginous  plates  of 
the  trachea  and  bronchi, 
the  larger  cartilages  of  the 
nose  and  the  middle  part 
of  the  Eustachian  tube. 
In  the  embryo  the  entire 
skeleton,  with  the  excep- 
tion of  part  of  the  skull,  is 
mapped  out  by  primarj^ 
hyaline  cartilage. 

The  intercellular  ma- 
trix is  apparently  homo- 
geneous, but  after  appropriate  treatment  it  is  resolvable  into  bundles  of 
white  fibres;  ordinarily,  however,  these  are  so  closely  united  and  blended  by 
the  cementing  ground-substance  that  the  presence  of  the  component  fibres 
is  not  evident.  The  cartilage-matrix  is  chemically  complex,  consisting  of  a 
mixture  of  collagen,  chondro-mucoid  and  albuminoid  substances. 

The  cartilage-cells,  as  the  connective  tissue  elements  within  the  matrix 
are  called,  are  irregularly  oval  or  spherical  nucleated  bodies.  They  are 
lodged  within  the  interfascicular  spaces,  or  lacuna;,  which  they  almost  or 
quite  fill.      In  adult  tissue  usually  two  or  more  cells  share  the  same  compart- 


^C' 


/T  - 


&* 


i^ 


IS 


\"^ 


/V 


perichondrium 


\  oting 
eartiUge-cells 


_Group  of  older 
cells 


-Cartilage-cells 


Lacuna  con- 
-tainiuK  nest  of 


Empty  lacuna 
-suriounded  by 
hyaline    matrix 


s>.  .  - 


^fx,   (^\  / 


Fig.  45. — Transverse  section  of  peripheral  portion  of  costal 
cartilage.     X  250. 


32  NORMAL   HISTOLOGY. 

vnent,  the  group  being  the  descendants  of  the  original  occupant  of  the  space. 
The  matrix  immediately  surrounding  the  lacunae  is  specialized  as  a  layer  of 
different  density,  which  is  described  as  a  capsule  ;  a  further  differentiation  of 
the  intercellular  substance  is  often  exhibited  by  the  more  recently  formed 
matrix,  which  often  stains  with  intensity  and  thereby  produces  the  local  ter- 
ritories known  as  the  cell-areas.  The  lacunae  of  hyaline  cartilage  are  homol- 
ogous with  the  lymph-spaces  of  other  dense  connective  tissues,  although 
channels  establishing  communication  between  the  adjacent  lacunae  are  not 
demonstrable  in  the  higher  vertebrates. 

The  free  surface  of  cartilage  is  covered  by  an  envelope  of  dense  connec- 
tive tissue,  the  perichondi'ium.  The  latter  consists  of  a  compact  external 
fibrous  layer  and  a  looser  inner  or  chondrogenetic  layer,  containing  many 
connective  tissue  cells.  These  elements  are  disposed  in  rows  parallel  to  the 
surface  of  the  cartilage,  and  during  the  peripheral  growth  of  the  tissue, 
gradually  assume  the  characteristics  of  cartilage  cells,  being  at  first  spindle- 
shaped  and  later  ovoid  and  spherical.  The  young  cartilage  cells  thus 
formed  become  gradually  separated  by  increasing  tracts  of  the  newly  depos- 
ited intercellular  matrix.  As  the  groups  of  cells  arising  from  the  division  of 
the  transformed  elements  recede  from  the  perichondrial  surface,  they  lose 
their  parallel  disposition  and  become  irregularly  arranged  and  further  sep- 
arated. In  addition  to  the  perichondrial  growth  at  the  surface,  cartilage 
also  increases  by  interstitial  growth  effected  by  the  formation  of  new  cells 
and  the  associated  matrix  in  the  interior  of  the  cartilage.  The  interstitial 
method  is  identified  with  the  expansion  of  the  primary  cartilages,  while  the 
perichondrial  one  is  conspicuous  in  bringing  about  the  additions  of  new 
cartilage  during  the  development  and  growth  of  the  long  bones. 

In  articular  cartilage  the  superficial  zone  contains  sparsely  distributed 
groups  of  small  cells  arranged  parallel  to  the  free  surface.  Within  the  deeper 
strata,  these  groups  are  replaced  by  elongated  rows  of  larger  elements  lying 
perpendicular  to  the  articular  surface.  This  columnar  disposition  of  the  carti- 
lage-cells is  particularly  evident  towards  the  underlying  epiphyseal  bone. 

In  parts  of  the  cartilage  remote  from  the  perichondrium,  the  matrix 
sometimes  exhibits  a  glistening  fibrous  structure;  more  often  patches  of 
opacity  and  granularity,  due  to  deposits  of  lime-salts,  affect  the  hyaline 
matrix.  Such  areas  of  calcification  are  common  in  the  tissues,  as  the  costal 
cartilages,  of  aged  subjects,  although  similar  changes  are  almost  always  pres- 
ent in  the  laryngeal  cartilages,  particularly  the  thyroid  and  the  cricoid,  as 
early  as  the  twentieth  year.  They  may  progress  until  complete  calcification 
of  the  cartilage  occurs.  Histologically  this  alteration  consists  of  a  deposit 
of  the  inorganic  material  within  the  matrix  and  is  not  osseous  tissue,  as 
implied  by  the  frequently  misapplied  term,  ' '  ossification. ' ' 

The  blood-vessels  of  cartilage  are  usually  limited  to  the  periphery, 
within  the  perichondrium  or  the  associated  synovial  membranes.  Nutrition 
of  the  cartilage  is  maintained  by  imbibition  of  the  tissue-juices  through  the 
matrix  into  the  lacuuce.  In  the  thicker  masses,  as  in  the  cartilages  of  the 
ribs,  nutrient  canals  exist  in  those  portions  most  remote  from  the  perichon- 
drium. Such  spaces  contain  a  small  amount  of  areolar  tissue  supporting  the 
blood-vessels;  the  latter,  however,  are  limited  to  th^  canals  and  the  nutri- 
tion of  the  cartilage-tissue  is  effected  here,  as  elsewhere,  by  absorption 
through  the  matrix.  The  lymphatics  are  sparingly  present  in  the  perichon- 
drium. N'erves  never  have  been  demonstrated  within  the  cartilages,  which 
fact  is  in  accord  with  the  insensibility  of  these  tissues  and  their  adaptation  to 
the  friction,  concussion  and  compressions  incident  to  their  functions. 


CARTILAGE. 


33 


Cartilage-cells 


Area  of  hyaline 
matrix  ' 


Network  of  elastic 
fibres 


Lacuna   containing 
cartilage-cell 


^. 


m    % 


Fig.  46. — Section  of  elastic  cartilage  from  the  epiglottis.    X  360. 


Elastic    cartilage,  called  also  yellozv  elastic  and  reticular  cartilage 
(Fig.  46),  has  a  limited  distribution,  occurring  principally  in  the  cartilages 
of  the   external  ear,   lower 
part  of  the  Eustachian  tube, 

and  in  parts  of  the  laryn.x,  .ja-  /  ' 

namely,  the  epiglottis,   the  .    '  -    ^ 

cuneiform    and    corniculate 

cartilages    and     the    vocal  r^^   . 

processes  of   the  arytenoid  ^-^  .v 

cartilages.  In  its  physical 
properties  this  variety  differs 
from  hyaline  cartilage,  as  it 
is  dull  yellowish  in  color  and 
pliable  and  tough  in  con- 
sistence, in  contrast  to  the 
bluish  opalescent  tint  and 
comparative  brittleness  of 
hyaline  cartilage. 

The  characteristic  his- 
tological feature  of  elastic 
cartilage  is  the  presence  of 
elastic  fibres  within  the  inter- 
cellular matrix.  The  lacunae 
containing  the  cartilage- 
cells  are  immediately  sur- 
rounded by  limited  areas  of 
hyaline    matrix,    the  so-called   "capsules"   of    some  authors.      The  matrix 

between    these    homogcne- 
.J,  . -,       --a.-^xi    -A.v--'-     ■       .sSi,.,,.  ous  areas,  however,  IS  pene- 

trated by  delicate  and  often 
intricate  networks  of  elastic 
fibres.  The  latter  respond, 
as  in  other  localities,  to  spe- 
cific stains.  The  method  by 
which  the  elastic  fibres  de- 
velop is  uncertain,  although 
their  production  must  be 
attributed  to  the  influence  of 
the  cells.  Since  the  fibres 
appear  relatively  late  and 
within  the  matrix  often  at 
some  distance  from  the  car- 
tilage-cells, it  is  probable 
that  they  do  not  arise  within 
the  exoplasm  of  the  cells. 
The  elastic  cartilages  are 
surrounded  by  a  perichon- 
drium of  the  usual  descrip- 
tion. 

Fibrous  cartilage,  or 
fibrocartilage  (Fig.  47),  is 
found  in  comparatively  few  localities,  although  it  occurs  in  masses  of 
considerable   bulk.      Its   chief  situations    are    the    intervertebral  disks,   the 


Hyaline 
area  sur- 
-rounding 
canilage- 
cells 


■;?v 


Fibrous  in- 

-teieeilular 
substance 


Cartilage- 
"cells 


■'^X 


Fig.  47. — Section  of  fibrous  cartilage  from  intervertebral  disk. 
X  225. 


34  NORMAL   HISTOLOGY. 

symphyses,  the  marginal  plates  and  interarticular  disks  of  certain  joints,  the 
sesamoid  cartilages  and  the  lining  of  bony  grooves  for  tendons.  In  its 
physical  properties,  this  tissue  combines  the  flexibility  and  toughness  of 
fibrous  tissue  with  the  firmness  and  elasticity  of  cartilage.  In  structure, 
fibrous  cartilage  resembles  dense  fibrous  co^necti^'e  tissue,  since  the  principal 
constituents  of  its  matrix  are  the  wavy  bundles  of  closely  packed  white  fibres. 
Between  these  bundles  lie  small  irregularly  distributed  oval  areas  of  hyaline 
matrix  (the  so-called  ' '  capsules' '  ),  which  immediately  surround  the  cartilage- 
cells,  singly  or  in  groups.  The  number  of  cells  and  the  proportion  of  fibrous 
matrix  differ  in  \'arious  localities.      A  distinct  perichondrium  is  wanting. 

BONE. 

Bone  or  osseous  tissue  is  a  dense  form  of  connective  tissue,  the  matrix 
of  w'hich  is  impregnated  with  lime-salts;  to  this  modification,  shared  by  the 
dentine  of  the  teeth,  is  due  the  characteristic  hardness  of  the  tissue.      In 


Fig.  48. — Section  of  frontal  bone,  showing  the  spongy  bone  enclosed  within  the  lamellated  compact  bone  ; 
the  latter,  however,  does  not  contain  Haversian  systems.     X  18. 

addition  to  forming  the  bones  of  the  principal,  and  in  man  the  only,  frame- 
work, ox .  eiido skeleton,  osseous  tissue  occurs  in  the  lower  vertebrates  asso- 
ciated with  the  integument  as  an  exoskeleton,  represented  by  the  dermal 
plates  of  the  crocodile  or  the  shell  of  the  turtle.  Within  various  organs,  as 
the  sclerotic  coat  of  the  eye  of  birds,  the  diaphragm  of  the  camel,  the  tongue 
of  certain  birds  or  the  snout  of  the  hog,  it  constitutes  the  splanchno skeleton. 
True  osseous  tissue  does  not  occur  outside  the  vertebrates,  the  skeletal 
frameworks  of  invertebrate  animals  consisting  of  calcareous  incrustations  or 
of  silicious  structures. 


BONE.  35 

Bone  consists  of  two  parts,  an  animal  and  an  earthy  portion,  the  former 
giving  toughness  and  the  latter  hardness  to  the  osseous  tissue.  The  animal  or 
organic  part  of  bone  may  be  removed  by  calcination,  the  inorganic  constitu- 
ents remaining  undisturbed.  After  such  treatment,  while  retaining  its  form, 
the  bone  is  fragile  and  easily  crushed  and  has  suffered  a  loss  of  one-third  of 
its  weight,  due  to  the  destruction  and  elimination  of  the  animal  matters.  The 
inorganic  material,  on  the  other  hand,  may  be  removed  by  the  action  of 
dilute  hydrochloric  acid,  which  dissolves  out  the  earthy  matters  and  leaves 
the  animal  part  intact.  Treated  in  this  manner,  the  bone,  although  re- 
taining perfectly  its  details  of  form,  is  tough  and  flexible,  a  decalcified  rib  or 
fibula  being  readily  tied  into  a  knot.  The  animal  constituents  yield  gelatin 
after  boiling  with  water,  consisting  chiefly  of  collagen  and  osseo-mucoid. 
The  inorganic  constituents,  which  form  approximately  two-thirds  of  the  bone, 
include  a  large  percentage  (83) of  calcium  phosphate,  much  less  calcium  car- 
bonate, with  small  proportions  of  calcium  fluoride  and  chloride,  and  of  the 
salts  of  magnesium  and  sodiuui. 

On  sawing  through  a  bone  from  which  the  marrow  and  other  soft  parts 
have  been  removed,  the  osseous  tissue  is  seen  to  be  arranged  as  a  peripheral 
zone  of  compact  bone  enclosing  a  \-ariable  amount  of  cancellated  bone.  In 
the  typical  long  bones,  as  the  humerus  or  femur,  the  compact  tissue  almost 
exclusively  forms  the  tubular  shaft  enclosing  the  large  marroiv-cavity ^  while 
the  cancellated  tissue  constitutes  the  expanded  extremities,  with  the  excep- 
tion of  a  narrow  superficial  stratum  of  compact  bone.  The  irregular  clefts 
between  the  lamellae  of  the  spongy  bone  are  direct  extensions  of  the  general 
medullary  cavity  and  are  filled  with  marrow-tissue.  In  the  flat  bones  of  the 
skull  (Fig.  48),  the  compact  substance  is  arranged  as  an  outer  and  an  inner 
plate,  or  tables^  of  considerable  thickness,  between  which  lies  the  spongy 
bone,  or  diplde.  The  short  and  irregular  bones  are  made  up  of  an  inner 
mass  of  spongy  bone  co\-ered  everywhere  by  a  shell  of  compact  substance, 
\Yhich  often  is  locally  thickened  to  insure  additional  strength  where  needed. 

The  cancellated  bone  consists  of  delicate  bars  and  lamelke  united  into 
an  intricate  osseous  reticulum  well  calculated  to  yield  strength  without 
undue  weight.  In  many  positions,  conspicuously  in  the  neck  of  the  femur, 
the  more  robust  lamellae  are  disposed  according  to  a  definite  plan  in  order 
to  meet  the  strains  of  pressure  and  of  tension.  Although  composed  of  the 
same  structural  elements,  compact  and  spongy  bone  clif?er  in  their  histo- 
logical details  in  consequence  of  the  secondary  modifications  which  take 
place  during  the  conversion  of  the  spongy  bone,  the  original  form,  into  the 
compact.  To  obtain  the  classic  picture  of  bone-tissue,  in  order  to  study  its 
general  arrangement  where  most  typical,  it  is  desirable  to  examine  thin 
ground  sections  of  the  compact  substance  cut  at  right  angles  to  the  axis  of 
a  long  bone  which  has  been  macerated  and  dried. 

The  compact  bone  in  such  preparations,  when  examined  under  low 
magnification  (Fig.  49),  is  seen  to  consist  of  osseous  layers  arranged  as 
three  chief  groups:  («)  circumferential  lamellce,  which  extend  parallel  to  the 
external  and  internal  surfaces  of  the  compact  bone;  {b)  Haversian  lamelltz, 
which  are  disposed  concentrically  and  form  conspicuous  annular  groups,  the 
Haversian  systems,  enclosing  the  Haversian  canals;  and  (<:)  interstitial  or 
ground  lamell(S^  which  include  the  irregularly  arranged  tracts  filling  the 
intervals  between  the  Haversian  systems  and  the  surface  lamellae. 

Each  Haversian  system  consists  of  the  concentrically  disposed  lamellae 
and  the  centrally  situated  channel,  the  Haversian  canal,  which  encloses  pro- 
longations of  the  marrow-tissue  and  ramifications  of  the  medullary  blood- 


36  NORMAL   HISTOLOGY. 

vessels.  Between  the  annular  lamellae  are  seen  small  spindle-shaped  or  oval 
spaces,  the  lacuncB  (about  20  m  long,  10  fj.  wide  and  6  'j.  thick),  from  which 
minute  passages,  the  cmialiculi,  radiate  and  join  with  others  to  establish 
communication  between  the  adjacent  lacunae  of  the  same  Haversian  system. 
The  lacunae  and  canaliculi  thus  form  an  intercommunicating  network  of 
lymph-spaces  similar  to  those  in  other  dense  connective  tissue.  When 
viewed  in  profile,  as  they  are  in  sections  cutting  the  lamellae  at  right  angles, 
the  lacunae  present  their  smaller  dimensions  and  appear  as  minute  fusiform 


External 
circumferential  lamellae 


-   —  ^-^  ^-^,  oA  -^.t-v  -  ^^^-%  -X  4  f ^^ 


Haversian  canal 
surrounded  by 
Haversian  lamellae 


Interstitial  lamellae 


Internal 
circumferential  lamellae 


Fig.  49.— Transverse  section  of  compact  bone;  the  section  has  been  ground  and  dried,  hence  the  lacunae 

are  filled  with  air.     X  70. 

spaces;  seen  in  sections  which  pass  parallel  to  the  lamellae  (Fig-  50)'  ^}^^ 
lacunae  are  broader  and  more  circular,  the  spaces  with  the  canaliculi  forming 
the  spider-like  figures  so  conspicuous  in  sections  of  dried  bone. 

The  characteristic  disposition  of  the  lamellae  of  the  Haversian  system  is 
due  to  the  secondary  formation  of  the  bone-tissue  during  the  convention  of 
the  spongy  bone  into  the  compact,  the  circumference  of  each  system  cor- 
responding to  an  Haversian  space,  the  cavity  in  which  the  connective 
lamellae  were  deposited.  It  follows,  from  this  relation,  that  the  Haversian 
systems  exist  only  in  compact  bone,  the  secondary  deposit  not  occurring 
during  the  development  of  cancellated  bone.     When  deprived  of  the  min- 


BONE. 


37 


eral  matters  and  examined  in  thin  fragments,  the  osseous  lamellae  often 
exhibit  indications  of  the  fibrous  structure  which  they  really  possess,  since 
the  bone- matrix  consists  of  closely  felted  bundles  of  white  fibres  united  by 


Circumferential 
lamellae 


Interstitial  lamellae 


Haversian  canal 


Obliquely  cut 
Haversian  canal 


FlO.  50. — Longitudinal  section  of  compact  bone,  ground  and  dried.     X  7°- 

cement-substance.  Within  the  Haversian  lamellae  the  fibrous  bundles  cross 
generally  at  right  angles,  but  within  the  other  lamelL-e  they  are  placed  less 
regularly  and  more  obliquely. 
On  examining  decalcified  bone, 
either  in  section  or  after  being 
pulled  apart,  bundles  of  fibrous 
tissue  are  seen  which  penetrate 
the  outer  circumferential  lamel- 
lae in  a  direction  perpendicular 
or  oblique  to  the  surface  and 
thus  pin  or  bolt  the  layers 
together.  Such  bundles,  the 
perforating  fibres  of  S/iarpey, 
are  numerous  in  the  lamellae 
beneath  the  periosteum,  from 
the  inner  layer  of  which  mem- 
brane they  are  derived.  The 
perforating  fibres  consist  of 
bundles  of  fibrous  tissue,  with  a  variable  number  of  elastic  fibres;  since  they 
are  often  imperfectly  calcified,  on  drying  they  leave  minute  canals  which 
pierce  the  lamellae  from  the  svirface  of  the  bone.      Being  produced  by  the 


Haversian 
canal' 

Lacuna  in 
profile 


Fig.  51. 


-Portion  of  adjacent  Haversian  system  cut  trans- 
versely.    X  250. 


38 


NORMAL    HISTOLOGY. 


Fig.  52. — Lacunae  and  canaliculi  in  dried  bone,  cut  par- 
allel with  the  lamellae.     X  300. 


periosteum,   Sharpey's   fibres   are   never    found    in   the    secondary    lamellae 
constituting  the  Haversian  systems. 

The  Haversian  canals  (.05-.  i   mm.  in  diameter)  are  continuations  of 
the  medullary  cavity  and  in  the  case  of  the  larger  ones,  contain  prolongations 

of  the  marrow-tissue.  They  serve 
the  important  purpose  of  carrying 
blood-vessels  into  the  interior  of 
the  compact  bone.  From  these 
\essels  the  nutritive  fluids  pass 
into  the  canaliculi  and  the  lacunae 
and  so  on  through  the  dense  tis- 
sue, the  nutrition  of  the  lamellae 
and  the  enclosed  bone-cells  being 
in  this  manner  insured.  The 
indi\idual  canals  are  short  and 
communicate  by  oblique  branches 
with  adjacent  channels.  They 
also  indirectly  communicate  with 
the  external  surface  of  the  bone  by  means  of  passages,  the  ^''olkmann  canals, 
within  the  circumferential  lamellae.  These  canals  open  on  the  surface  and 
convey  twigs  from  the  periosteal  blood-vessels  into  the  lamelhe  other  than 
those  of  the  Haversian  systems.  The  twigs  entering  by  the  superficial  canals 
freely  anastomose  with  those  within  the  Haversian  canals,  the  compact  bone 
being  thus  provided  with  a  vascular  network  derived  from  both  sources. 

The   bone-cells  are   connective   tissue   elements  imprisoned  within   the 
lacuncC,  an  arrangement  similar  in  principle  to  that  within  the  cornea  where 
the  corneal  cells  lie  within  the  lymph-spaces  of  the  ground-substance.      Sec- 
tions of  dried  bone,  useful  as  they  are  in  aftording  striking  pictures  of  general 
arrangement,  are  entirely  inadequate  for  the  study  of  the  bone-cells,  since 
the  latter  are  shrunken  and  lost  in   the  debi^is  which,    with  air,    fills  the 
lacunae  in  the  ground  specimens.      In  order  to  exhibit  the  bone-cells,  after 
fixation  the  tissue  is  decalcified,  sectioned  and  stained  and  mounted  in  an 
approved  preser\-ing  medium.      By  such  treatment  the  integrity  of  the  cells 
is  insured,  although  the  lacunae  and  canaliculi  no  longer  show  with  diagram- 
matic sharpness  in  consequence  of  being  permeated  with  the  mounting  medi- 
um instead  of  air.      The  bone-cells,  after  being  stained  in  such  decalcified 
sections,  appear  as  small  lenticular  or  stellate 
bodies  within  the  lacunae,  which  they  almost 
or  quite  fill  ("Fig.  53).     The  deeply  tinged  nu- 
cleus shows  as  a  brilliant  dot  within  the  lighter  .  . .:, 
andfaintly  granular  cytoplasm,  which  extends                >^%,             ''     --;        "'< 
from  the  stellate  cell-body  into  the  canaliculi 
as  delicate  processes  of  variable  length. 

The  Periosteum. — The  external  sur- 
face of  bones  is  closely  invested,  except  when 
covered  with  cartilage,  by  a  fibrous  mem- 
brane, the  periosteiLin,  a  structure  of  great 
importance  during  development  and  growth, 
and  later  for  the  nutrition  and  repair  of  the 
osseous  tissues.  The  adult  periosteum  consists  of  two  layers,  an  oxxl^x fibrous 
and  an  inner  fibro-elastic ;  during  periods  of  growth,  an  additional 
stratum,  the  osteogenetic  layer,  lies  next  and  closely  related  with  the  exterior 
of  the  bone. 


''^^. 


53. — Bone-cells    lying  within   the 
lacunae.     X  530. 


THE  PERIOSTEUM. 


39 


The  fibrous  layer  is  composed  of  bundles  of  fibrous  tissue  and  sup- 
ports the  larger  blood-vessels  which,  within  the  deeper  parts  of  the  perios- 
teum, break  up  into  twigs  that  enter  the  surface  of  the  bone  through  the 
Volkmann  canals.  The  fibro -elastic  layer  includes  a  feltwork  of  elastic 
fibres  and  delicate  strands  of  fibrous  tissue.  The  inner  surface  of  the  perios- 
teum is  attached  to  the  underlying  bone  by  processes  of  connective  tissue 
which  accompany  the  blood-vessels  into  the  superficial  canals.     This  relation 


f:  Mr 


Last  formed  lamella 
of  bone 


-Dense  fibrous  layer 


Marrow-tissue  con- 
tinuous with  perios- 
teum 


Periosteal  blood-ves- 
sel passing-  into  the 
bone 


Bone-cell  within 
lacuna 


smmmm 


ii|;^.       Remains  of  osteoge- 
'"'      netic  layer 


'"^MP:'.. 


um  4' 


Fig.  54. — Section  of  young  periosteum  and  subjacent  bone.     X  275. 

persists  from  the  continuity  of  the  formative  tissue  of  the  young  periosteum 
with  the  early  marrow- tissue. 

The  osteogenetic  layer,  during  development  and  growth  of  the  bone, 
consists  of  delicate  bundles  of  fibrous  tissue  and  large  numbers  of  round  or 
fusiform  connective  tissue  cells  of  an  embryonal  type.  Those  next  the 
growing  bone  are  of  irregular  cuboid  form  and  disposed  in  a  single  row  upon 
the  surface  of  the  developing  osseous  tissue.  Since  these  cells  are  directly 
concerned  in  producing  the  new  bone,  they  are  termed  osteoblasts.  Later 
some  of  them  become  imprisoned  within  the  bone-matri.\  and  transformed 
into  bone-cells.  After  completion  of  its  active  role,  the  osteogenetic  layer 
becomes  greatly  reduced  and  inconspicuous,  in  the  adult  periosteum  being 
represented  by  flattened  cells,  which  no  longer  form  a  continuous  stratum  but 
occur  as  scattered  groups. 

In  addition  to  its  important  bone-producing  function,  the  periosteum 
serves  as  the  immediate  means  by  which  muscles,  tendons,  ligaments  and 
fasciae  gain  attachment  to  the  skeleton.  In  every  case  the  union  is  effected 
by  the  fusion  and  blending  of  the  connective  tissue  of  the  attached  structure 
with  the  outer  layer  of  the  periosteum. 

Bone -Marrow. — The  spaces  in  the  interior  of  bones,  whether  the  large 
medullary  cavities  surrounded  by  the  compact  substance  forming  the  tubular 


40  NORMAL   HISTOLOGY. 

shaft  of  the  long  bones  or  the  irregular  interstices  between  the  trabeculae  of 
the  cancellated  tissue,  are  filled  with  bone-marrow.  The  latter  also  extends 
into  the  larger  Haversian  canals.  Apart  from  its  fundamental  relations  to  the 
development  of  bone,  in  addition  to  supporting  the  medullary  blood-vessels, 
and  therefore  assisting  in  maintaining  the  nutrition  of  the  bone,  the  marrow 
plays  a  very  important  role  in  connection  with  the  production  of  blood-cells. 
Indeed,  with  its  chief  functions  in  mind,  bone-marrow  is  classed  as  a  blood- 
formi7ig  organ,  and,  as  such,  finds  its  systematic  consideration  with  the 
blood.  As  a  matter  of  convenience,  however,  a  general  description  of  the 
marrow-tissue  is  here  given,  while  the  relations  of  its  cellular  elements  to 
the  circulation  are  discussed  in  connection  with  the  Blood  (page  lOo). 

Although  of  a  reddish  tint  within  all  the  bones  of  the  early  skeleton,  the 
marrow  in  the  adult  includes  two  kinds—  the  red  and  the  yellow.  Thus, 
within  the  shaft  of  the  long  bones  it  appears  as  a  light  yellowish  tissue,  pre- 
senting the  characteristics  of  ordinary  adipose  tissue;  while  within  the  upper 
ends  of  the  humerus  and  of  the  femur,  and  especially  within  the  bodies  of 
the  vertebrae,  the  ribs, the  sternum  and  the  diploe  of  the  cranium,  the  marrow 
possesses  a  dull  red  color. 

Red  Marrow. — The  ingrowth  of  the  periosteal  tissue  and  blood-vessels 
constitutes  the  primary  marrow  of  the  foetal  skeleton;  from  this  tissue  the 
red  marrow  filling  the  young  bones  is  directly  derived.  The  red  marrow  is, 
therefore,  the  first  formed  and  typical  variety.  After  early  childhood,  how- 
ever, the  marrow  within  the  bones  of  the  extremities  suffers  gradual  invasion 
by  fat,  until,  with  the  exception  of  the  marrow  within  the  upper  ends  of  the 
humerus  and  of  the  femur,  the  red  tissue  gives  way  to  the  yellow,  the  fat- 
cells  replacing  most  of  the  marrow-elements. 

When  examined  in  section  after  fixation  and  appropriate  staining,  the 
red  marrow  exhibits  a  delicate  connective  tissue  reticulum  which  supports 
the  blood-vessels  and  contains  within  its  meshes  numerous  cells  (Fig.  55). 
Next  the  bone,  the  fibrous  tissue  forms  a  thin  membrane,  the  endostetan, 
lining  the  medullary  cavity  and  extending  into  the  larger  Haversian  canals. 
The  more  characteristic  cells  encountered  within  the  red  marrow  include: — 
(i)the  myelocytes,  very  numerous  and  of  different  ages;  (2)  the  eosinophiles ; 
(3)  the  megakaryocytes  or  giant  cells  and  (4)  the  erythroblasts  and  other 
stages  of  red  blood-cells.  In  addition  lymphocytes,  connective  tissue  cells, 
fat-cells,  mast-cells  and  osteoclasts  are  usually  present  in  small  and  varying 
numbers. 

The  myelocytes  include  three  varieties  of  cells  which,  while  differing 
in  the  granularity  of  their  cytoplasm  and  the  form  of  their  nuclei,  are 
probably  directly  genetically  related.  («)  The  -myeloblasts  are  relatively  large 
ovoid  cells,  with  round  nuclei  and  cytoplasm  devoid  of  granules.  They  are 
few  in  number  and  regarded  as  the  parent  elements  (premyelocytes).  (^) 
The  myelocytes,  the  most  numerous  of  the  marrow-cells,  are  of  uncertain 
form  and  possess  large  round  nuclei  and  cytoplasm  containing  fine  neutro- 
philic granules,  (c)  The  polymorphonuclear  leucocytes,  the  descendants  of 
the  preceding  elements,  are  somewhat  smaller  and  more  granular  than  the 
myelocytes  and  distinguished  by  the  lobulated  nuclei  so  conspicuous  in  the 
most  common  form  of  colorless  blood-cells.  If  the  genetic  relations  here 
outlined  be  accepted,  the  typical  leucocytes  represent  later  generations  of 
the  marrow-cells,  which  thus  become  important  contributors  to  the  blood. 
The  eosinophiles,  sparingly  represented,  are  conspicuous  by  reason  of  the 
coarse  granules  within  their  cytoplasm  which  color  intensely  with  acid  stains 
(eosin).      The  relation  of  the  eosinophiles  to  the  myelocytes  shares  the 


RED  BONE-MARROW.  41 

uncertainty  of  their  position  as  blood-elements.  The  megakaryocytes, 
the  mononuclear  giant  cells  of  the  marrow,  are  huge  and  conspicuous  occu- 
pants of  the  reticular  meshes.  They  must  not  be  confused,  however,  with 
giant  cells  of  another  kind,  the  osteoclasts,  which  in  young  marrow  are 
more  numerous  and  equally  striking.  The  distinguishing  feature  between 
the  two  is  the  nucleus,  which  in  the  case  of  the  osteoclast  is  multiple  and  in 
that  of  the  megakaryocyte  single.  In  the  last  instance,  however,  the  nucleus 
may  assume  a  very  complex  contour,  sometimes  being  so  lobulated  and  con- 


Myelocytes 


Nucleated  red_ 
blood- eel  Is 


*  O  Giant  cell 


Giant  ceH- 


Blood-vesse, 


Blood-vessel 


Reticulum 


Fig.  55. — Section  of  red  bone-marrow  from  end  of  young  femur.     X  300. 

torted  that  what  is  really  one  continuous  nucleus  appears  as  several.  The 
osteoclasts  are  multinuclear,  two  or  more  ovoid  nuclei  occupying  the  huge 
mass  of  granular  cytoplasm.  These  cells,  moreover,  lie  close  to  the  trabeculae 
of  young  bone,  a  position  in  keeping  with  their  particular  function  as  bone- 
destroyers.  The  origin  and  role  of  the  megakaryocytes  are  uncertain,  a 
derivation  from  the  myelocytes  and  a  function  as  phagocytes  or  as  producers 
of  the  blood-plates  (Wright)  being  among  the  suggested  explanations  con- 
cerning these  constituents  of  the  marrow. 

The  nucleated  red  blood-cells  are  constant  elements  of  the  red 
marrow  and  indicate  its  importance  as  the  chief  seat  in  which  the  production 
of  the  red  cells  after  birth  takes  place.  These  blood- cells  are  represented 
by  three  generations:  {a)  the  erythroblasts,  the  descendants  of  the  primary 
blood-cells,  which  are  transformed  into  {b^  the  normoblasts.  The  latter  are 
smaller  than  the  erythroblasts  and  po.ssess  nuclei  in  which  the  reticular 
structure  has  given  place  to  one  of  density.  By  mitotic  division  the  normo- 
blasts give  rise  to  (^)  the  nucleated  erythrocytes,  which,  after  losing  their 
nuclei,  become  the  completely  developed  erythrocytes  and  pass  into  the 
circulation  as  the  ordinarv  red  blood-cells. 


42  NORMAL    HISTOLOGY. 

Yellow  Marrozv. — After  early  childhood,  the  red  marrow,  which  pre- 
viously fills  all  the  medullary  spaces,  begins  to  suffer  invasion  by  fat-cells 
and  conversion  into  adipose  tissue.  This  change,  which  results  from  the 
substitution  of  fat-cells  for  the  distinctive  marrow  components,  affects  the 
medullary  tissue  within  the  bones  of  the  extremities,  except  within  the  upper 
ends  of  the  humerus  and  femur.  Examined  in  section,  the  yellow  marrow 
resembles  ordinary  adipose  tissue,  consisting  chiefly  of  the  large  compressed 
spherical  fat-cells  supported  by  a  reticulum  of  connective  tissue.  The  cells 
belonging  to  the  latter,  occasional  plasma  cells  and  myelocytes  are  the 
usual  elements  encountered.  In  advanced  age  and  during  starvation,  the 
customary  consistence  and  yellow  color  give  place  to  a  mucoid  condition 
and  reddish  tint,  the  fat- cells  of  such  gelatinous  marroiv,  as  it  is  termed, 
losing  much  of  their  oily  contents. 

Blood-Vessels  of  Bones. — The  generous  blood-supply  of  bones  is 
arranged  as  two  sets,  the  periosteal  and  the  medullary.  The  former  consti- 
tutes a  network  within  the  periosteum  and  supplies  twigs,  which  enter  the 
subjacent  compact  bone  through  the  Volkmann  canals  and  communicate 
with  the  branches  from  the  medullary  system.  The  medullary  artery  is 
often,  as  in  the  case  of  the  long  bones,  a  vessel  of  considerable  size,  which, 
accompanied  by  the  companion  veins,  traverses  the  oblique  nutrient  canal 
to  gain  the  centre  of  the  marrow.  On  reaching  this  position  the  medullary 
artery  usually  divides  into  ascending  and  descending  branches,  from  which 
twigs  radiate  towards  the  periphery  of  the  marrow  cavity.  The  twigs  termi- 
nate in  arterial  capillaries,  which  expand  rather  abruptly  into  larger  venous 
capillaries,  in  consequence  of  this  arrangement  the  rapidity  of  the  blood- 
stream becoming  diminished  in  its  course  through  the  marrow.  Within  the 
red  marrow,  the  venous  capillaries  possess  an  imperfect  endothelial  lining, 
thereby  affording  an  opportunity  for  the  newly  formed  blood-cells,  the  eryth- 
rocytes and  the  leucocytes,  to  gain  entrance  into  the  circulation.  After  thus 
coming  into  close  relations  with  the  marrow-tissue,  the  blood  is  collected  by 
the  capillaries  which  form  veins  destitute  of  valves.  In  addition  to  the  com- 
panion veins  which  accompany  the  medullary  artery  through  the  nutrient 
canal,  in  many  instances  the  larger  veins  pursue  an  independent  course  and 
emerge  from  the  cancellous  tissue  by  means  of  special  canals  piercing  the 
compact  substance. 

Definite  lymphatics  are  found  only  within  the  outer  layer  of  the  perios- 
teum, although  the  system  of  intercommunicating  spaces  within  the  bone,  the 
lacunae,  and  canaliculi,  stands  in  close  relation  with  the  larger  lymph-channels. 

The  nerves  supplying  the  bones  include  both  meduUated  and  nonmed- 
ullated  fibres.  The  latter,  distributed  partly  to  the  periosteum  and  partly 
within  the  bone,  are  chiefly  sympathetic  fibres  destined  for  the  control  of  the 
involuntary  muscle  within  the  walls  of  the  blood-vessels.  The  medullated 
sensory  fibres  are  few,  some  being  connected  with  special  endings  of  the 
lamellar  type  within  the  periosteum. 

DEVELOPMENT   OF    BONE. 

With  the  exception  of  certain  parts  of  the  skull — the  vault  and  nearly 
all  of  the  face — the  bones  of  the  human  skeleton  are  preceded  by  solid 
masses  of  embryonal  hyaline  cartilage.  Since  the  primary  development  of 
such  bones  takes  place  within  the  cartilage,  they  are  known  as  cartilage 
bo7ies  and  the  mode  of  formation  is  termed  endochondral  development.  The 
bones  not  preceded  by  cartilage  are  produced  within  sheets  of  connective 


DEVELOPMENT   OF    BONE. 


43 


^-rr'-T-rS^Fi':;; 


tissue;  such  are  said,  therefore,  to  be  membrane  bones  and  their  formation  is 
bv  intramembranous  development. 

Endochondral  Bone-Development.  —The  process  by  which  bones 
[M-eceded  by  cartihige  are  formed,  known  as  the  endochondral  mode, 
inchides  two  distinct,  although  closely  related,  series  of  changes.  The  one 
results  in  the  production  of  osseous  tissue  within  the  mass  of  cartilage,  the 
intracartilaginoits  bone,  the  other  in  the  production  of  bone  outside  the 
cartilage  and  beneath  the  periosteum,  the  subperiosteal  bone.  Both  take 
part  in  the  formation  of  the  completed  bone,  although  their  contributions 
to  the  final  result  are  not  only 
unequal,  but  vary  with  differ- 
ent types  of  bones. 

The  greater  part  of  the 
bone  formed  within  the  carti- 
lage undergoes  absorption, 
the  spongy  substance  within 
the  ends  of  the  long  and  the 
bodies  of  the  irregular  bones 
chiefly  representing  the  per- 
sisting contribution  of  the  in- 
tra cartilaginous  bone.  Even 
when  the  intracartilaginous 
changes  are  conspicuous,  as 
in  the  development  of  the  hu- 
merus, femur  and  other  long 
bones,  the  important  compact 
substance  is  the  product  of 
the  periosteal  connective  tis- 
sue and  genetically  resem- 
bles intramembranous  bone. 
Although  the  formative  proc- 
esses of  both  kinds  of  bone 
proceed  coincidently  and  are 
closely  related,  as  a  matter  of 
convenience  they  will  be 
described  separately  and  as 
occurring  in  the  development 
of  a  typical  long  bone. 

Intracartilaginous  Bone. 
— The    primary  cartilage, 

formed  by  the  proliferation  and  condensation  of  the  mesenchymal  tissue, 
gradually  assumes  the  characteristics  of  embryonal  cartilage,  which  by  the 
end  of  the  second  month  of  foetal  life  maps  out  the  principal  segments  of 
the  skeleton.  These  segments  are  covered  by  an  immature  form  of  peri- 
chondrium, or  primary  periosteiim,  from  which  arise  the  elements  actively 
engaged  in  the  production  of  the  bone-tissue.  The  primary  periosteum 
consists  of  a  compact  outer  fibrous  and  a  loose  inner  osteogenetic  layer,  the 
latter  being  rich  in  cells  and  delicate  intercellular  fibres. 

The  initial  changes  within  the  cartilage  appear  at  points  known  as 
centres  of  ossification,  which  in  the  long  bones  are  situated  about  the  middle 
of  the  future  shaft.  These  early  changes  involve  both  cells  and  matrix, 
whicn  exhibit  conspicuous  increase  in  size  and  amount  respectively.  The 
cartilage-cells  become  larger  and  more  vesicular  (Fig.  56)  and  encroach  upon 


Embryonal 
cartilage 


Cartilage-cells 
becoming  enlarged 
and  regrouped 


Enlarged  cartilage- 
cells  at  centre  of 
ossification 


Periosteum 


Fig.  56. 


-Section  of  tarsal  bone  of  foetal  sheep,  showing  centre 
of  ossification.    X  50. 


44 


NORMAL   HISTOLOGY. 


Etnbryon 
cartilagi 


Young  periosteut 


Cartilage-cells  becotnine 
enlarged  and  grouped 


lit-:  f  "- i-;"« : ;-f.-V# 


Zone  of  calcification 


Osteognetic  layer  of 
periosteum 


Central  spongy  bone 

enclosing  remains  of 

cartilage 


:^:^K^;  y^'/ 


Fio.  57. — Longitudinal  section  of  metatarsal  bone  of  foetal  sheep,  showing  endochondral  bone- 
development.     X  40. 


DEVELOPMENT   OF   BONE.  45 

the  intervening-  matrix,  in  which  a  deposit  of  Hnie  salts  now  takes  place,  as 
shown  by  opacity  and  by  the  grittiness  when  a  knife  is  carried  through  such 
ossihc  centres.  On  acquiring  their  maximum  size,  the  cartilage-cells  soon 
give  indications  of  impaired  vitality  in  their  shrinking  cytoplasm  and  degen- 
erating nuclei.  The  enlarged  spaces  or  lacunae  enclosing  these  cells  are 
known  as  the  primary  arcolcs. 

Coincidently  with,  or  indeed  preceding,  these  changes  within  the  car- 
tilage, a  thin  peripheral  layer  of  bone  has  been  formed  beneath  the  young 
periosteum,  thickest  around  the  middle  of  the  shaft  and  fading  away  towards 
the  ends.  Bud-like  processes  of  the  osteogenetic  layer  grow  inward  from 
the  periosteum  and  invade  the  embryonal  cartilage,  by  absorption  of 
the  cartilage-matrix  gaining  the  centre  of  ossitication  and  there  effecting 
destruction  of  the  less  resistant  cells  and  the  intervening  matrix.  In  conse- 
quence of  this  invasion  by  the  periosteal  tissue,  a  space,  the  primarv 
marrou'-cavity,  now  occupies  the  centre  of  ossification  and  contains  the 
direct  continuation  of  the  osteogenetic  layer.  This  tissue,  the  primary 
marrow,  which  has  thus  gained  the  interior  of  the  cartilage,  contributes  the 
cells  upon  which  a  double  role  devolves — to  remo\-e  the  embryonal  cartilage 
and  to  produce  bone. 

The  cartilage-matrix  closing  the  enlarged  cell-spaces  on  the  side 
towards  the  primary  marrow-cavity  progressively  suffers  absorption,  whereby 
the  spaces  are  opened,  converted  into  secondary  areola:,  and  brought  into 
direct  communication  with  the  expanding  medullary  cavity.  The  car- 
tilage-cells escape  from  their  former  homes  into  the  marrow-cavity  and 
undergo  disintegration,  taking  no  part  in  the  direct  production  of  the 
bone-tissue. 

Beyond  the  immediate  limits  of  the  primary  marrow-cavity,  the  car- 
tilage cells  in  their  turn  undergo  the  increase  in  size  and  the  impairment  of 
vitality  described;  in  addition  they  often  exhibit  a  conspicuous  rearrange- 
ment, forming  columnar  groups  separated  by  intervening  tracts  of  calcified 
matrix  (Fig.  57).  This  characteristic  belt,  the  zone  of  calcification,  sur- 
rounds the  medullary  cavity  and  marks  the  area  in  which  the  destruction  of 
cartilage  is  progressing  with  greatest  energy.  In  consequence  of  the  dispo- 
sition of  the  cartilage  elements  as  columnar  groups  separated  by  intervening 
tracts  of  calcified  matrix,  a  less  and  a  more  resistant  portion  of  the  cartilage 
are  offered  to  the  attacks  of  the  marrow-tissue  by  the  cell-  and  the  matrix- 
columns  respectively.  As  the  result  of  this  difference,  the  cells  and  the 
immediately  surrounding  partitions  first  succumb,  while  the  intercolumnar 
tracts  of  calcified  matrix  remain  for  a  time  as  irregular  indented  tapering 
processes,  deeply  tinted  in  sections  stained  with  hematoxylin,  which  extend 
beyond  the  last  row  of  cartilage-cells  into  the  medullary  cavity.  These 
trabecnlae  of  calcified  cartilage-matrix  serve  as  supports  for  the  marrow-cells 
engaged  in  producing  the  true  bone,  these  elements,  the  osteoblasts,  becom- 
ing arranged  along  the  trabeculae  upon  which,  through  the  influence  of  the 
cells,  the  immature  bone-tissue  is  deposited. 

Simultaneously  with  the  destructive  phase  attending  the  absorption  of 
the  embryonal  cartilage  by  the  chondroclasts,  the  constructive  process  of 
bone-formation  is  instituted  by  the  osteoblasts.  The  osteoblasts  rest  on  the 
irregular  trabeculae  of  calcified  cartilage  and  bring  about  the  deposit  of  a 
layer  of  bone-matrix  upon  the  surface  of  the  trabeculae,  which  thus  becomes 
encased  within  a  shell  of  immature  bone.  After  the  latter  has  attained  a 
thickness  at  least  equal  to  that  of  the  osteoblasts,  some  of  the  latter  are  grad- 
ually surrounded  by  the  osseous  matrix,  until,  finally,  they  lie  isolated  within 


46 


NORMAL    HISTOLOGY. 


Imprisoned  osteo- 
blast becoming-  a 
bone-cell 


Osteoblasts^ 


Bone-cell- 


Fig.  58. — Portion  of  developing  osseous 
trabecula  and  osteoblasts.    X  350. 


the  newly  formed  bone  as  its  cells  (Fig.  58).  The  bone-cells  are,  therefore, 
imprisoned  osteoblasts,  which,  in  turn,  are  specialized  connective  tissue  ele- 
ments.     The   bone-cells   occupy   minute   lenticular   or  stellate  spaces,    the 

pj'imajy  lamncE,  at  this  stage  the  canaliculi 
being  still  unformed.      The  bone-matrix  is 
\  at  first  devoid  of   calcareous  material  and 

1/^  '^'  IS,  therefore,  soft;  very  soon,  however,  the 

deposit  of  lime-salts  begins  and  the  young 
bone  becomes  hard.  The  increase  of  the 
new  bone  is  attended  by  the  gradual  disap- 
pearance of  the  enclosed  calcified  cartilage- 
matrix,  the  last  traces  of  which,  however, 
persist  for  some  time  as  irregular  deepdy 
stained  patches  within  the  osseous  trabec- 
ule;, at  some  distance  from  the  zone  of 
calcification  (Fig.  57).  Many  of  the  newly 
formed  bony  trabeculse  soon  undergo  ab- 
sorption, with  corresponding  enlargement 
of  the  intervening  marrow- spaces.  The 
remaining  trabeculae  increase  in  thickness 
by  the  addition  of  new  lamellae  on  the  surface  covered  by  the  osteoblasts 
and  join  to  form  a  trabecular  network,  the  pj'hnmy  central  spongy  bone. 
In  the  irregular  bones,  the  primary  spongy  bone  is  represented  by  the 
cancellated  tissue  forming  the  internal  framework.  In  the  long  bones,  the 
primary  spongy  bone  undergoes  further  absorption  within  the  middle  of  the 
shaft,  simultaneously  with  its  continued  development  within  the  cartilage  at 
the  ends  of  the  shaft,  or  diaphyses.  As  the  result  of  this  absorption,  a  large 
space,  the  central  marrow  -  cavity  ^  is  formed,  the  growth  of  which  keeps 
pace  with  the  general  expansion  of  the  bone.  So  long  as  a  long  bone 
increases  in  length,  new  cartilage  is  added  at  the  ends  of  the  shaft,  to  be 
replaced  in  its  turn  by  the  ad- 
vancing osseous  tissue. 

The    absorption    of    the  '''^\ 

newly  formed    bone   is  effected  C 

through    the   agency    of    large  ';     v 

polynucleated    cells,    the    osteo-  v  :f^;" 

clasts.  These  are  specialized 
marrow-elements  whose  partic- 
ular role  is  the  breaking  down 
and  absorption  of  the  bone- 
matrix.  They  are  very  large 
(50—100  ,a)  and  lie,  singly  or  in 
groups,  close  to  the  surface  of 
the  bone  within  depressions, 
the  so-called  How  ship' s  lacjcncs, 
produced  by  the  erosion  of  the 
osseous  matrix  (Fig.  59).  The 
only  part  of  the  central  spongy 
bone  which  persists  after  the 
completed  development  and 
growth  of  the  long  bones,  is  that  constituting  the  cancellated  tissue  within 
their  ends.  It  will  be  seen,  therefore,  that  in  many  cases  the  product  of 
bone-formation  within  the  cartilage,  the  primary  central  spongy  bone,  is  to  a 


Howship's 
lacuna 


Osteoblasts  ^— — 


-'Osteoclast 


Bone-cell 

within 

lacuna 


w 


Fig.  59. — Portion  of  trabecula  of  spongy  bone  undergoing 
absorption  by  osteoclast.    X  450. 


DEXELOPMKXT    OF    BONE. 


niarrow-cavity 
spongy  bone. 


Endochondral 

spongj-  bone 


large  extent  absorbed  and  contributes  only  a  small  part  of  the  mature 
skeleton.  The  early  marrow-cavity,  including  all  its  ramifications  between 
the  trabecuke,  is  filled  with  the  young  marrow-tissue;  the  latter  gives  rise 
to  the  red  marrow  that  for  a  time  fills  all  the  bones  and  later  occupies  the 
spongy  tissue  chiefly  within  the  axial  skeleton.  It  may  be  emphasized,  that 
the  process  sometimes  sjjoken  of  as  the  "ossification  of  cartilage"  is  reallv 
a  substitution  of  osseous  tissue  for  cartilage  and  that,  even  in  the  endo- 
chondral mode  of  formation,  cartilage  is  ne\er  directly  converted  into  bone. 

Ossijicafioii  icithin  the  epiphyses,  which  usuallv  does  not  begin  until 
some  months  after  birth,  repeats  in  its  essential  features  the  details  of 
intracartilaginous  bone-formation  as  seen  in  the  development  of  the  shaft. 
After  the  establishment  of  the  primary 
and  the  surrounding 
ossification  extends  in 
two  directions — towards  the  periphery 
and  towards  the  adjacent  end  of  the 
shaft.  As  this  process  progresses,  the 
layer  of  cartilage  between  the  central 
spongy  bone  and  the  free  surface,  on 
the  one  hand,  and  between  the  central 
spongy  bone  of  the  epiphysis  and  that 
of  the  shaft,  on  the  other,  is  gradually 
reduced  until  in  places  it  entirely  dis- 
appears. Over  the  areas  which  cor- 
respond to  the  later  joint-surfaces, 
the  cartilage  persists  and  becomes  the 
articular  cartilage  covering  the  ends 
of  the  bone.  With  the  final  absorption 
of  the  plates  separating  the  epiph\-ses 
from  the  shaft,  the  osseous  tissue 
composing  the  segments  becomes  con- 
tinuous, ' '  bony  union' '  being  then 
accomplished. 

Subperiosteal  Bone.  — It  is  e\ident 
from  the  foregoing  account  of  the 
development  of  bone  within  cartilage 
that  the  true  bone-producing  elements 
are  contributed  by  the  periosteum 
when  the  latter  sends  its  processes  into 
the  ossific  centre  within  the  cartilage. 
The  distinction,  therefore,  between  intracartilaginous  and  subperiosteal  bone 
is  one  of  situation  rather  than  of  inherent  difference,  as  in  the  production 
of  both  the  osteoblasts  are  the  active  agents  and  the  essential  features  are 
identical.  .Since  in  the  development  of  subperiosteal  or  perichondral  bone 
the  changes  within  cartilage  do  not  come  into  account,  the  details  are  less 
complicated  and  concern  primarily  only  a  formati\e  process. 

The  young  periosteum,  it  will  be  recalled,  consists  of  an  outer  compact 
fibrous  layer  and  an  inner  loose  osteogenetic  layer.  The  latter  is  rich  in 
blood-vessels  and  contains  numerous  embryonal  connecti\-e  tissue  cells  and 
delicate  strands  of  fibres.  Some  of  these  cells  become  the  osteoblasts  and 
as  such  are  arranged  along  the  fibrillae,  about  which  the  bone-matrix  is 
deposited  through  the  influence  of  the  cells.  The  osseous  trabeculse,  formed 
in  this  manner  beneath  the  periosteum,  increase  in  length  by  the  addition  of 


Endochondral 

spongy  bone 

Zone  of 

calcification 


Embryonal 
cartilage 


V. 


Fig.    6o. — Longitudinal    section    of    phalanx   of 
foetus  of  five  months.     X  25. 


48  NORMAL    HISTOLOGY. 

new  matrix  at  the  periosteum,  and  in  thickness  by  the  deposition  of  new 
layers  of  bone-matrix  on  the  surface  of  the  trabeculae  by  the  osteoblasts, 
some  of  these  cells  being  surrounded  by  the  matrix  and  thus  converted  into 
bone-cells.  The  spaces  between  the  trabeculae  are  occupied  by  the  primary 
niarrow,  the  direct  prolongation  from  the  periosteal  tissue.  During  their 
further  growth,  the  trabeculae  unite  to  form  a  subperiosteal  bony  network, 
the  peripheral  spongy  bone,  which  surrounds  the  central  spongy  bone,  or, 
where  that  has  already  disappeared,  the  central  marrow  cavity.      Towards 


''■.      *l    -a  -  , 

V               » 

•v 

--  " 

* 

a   ?V  " 

,..",.  —   ^ ' 

^•-   '      ^ '^  [\~  *s.*-^^"  »,'M^*'   ^        j"    ""^"^ '?**•*•'•';•.*• '"^^'' 


■^1    ".'/.': 


,'  V,       bone 


Periosteum 


Osteogenetic 
layer 


Osteoblasts 


Last  formed 


''^M       "     '^    '"''"■V||^  .-<"»,    ^^..       ^   ^-i-   \     /-'s^^'"-^^    — ';*:     — Osteoblasts 
'C^^-y''    ^^\:\    ''^.-\ ^-1^   '      ^ Primary 


marrow 


^^:^%' 


^j\_>-  /       \      •   (  /■*    ^jfT^ — ,;_     "..<D ^  Central  bone 

'  Xs^'i'  ^   -"  ~    '    "  • .  *«i 

f        ''l&l^  *"         rs  '^  Remains  of 

/    !'/#■"  "'^  ■''"teS^^ W^ ' ■  ■"     >     .calcified 

^.        V  ''}lW'-'^y'-'\"       ^^,^#/'         ,    .   ^  '^  cartilage 

Fig.  6i.— Portion  of  developing  humerus  of  foetal  sheep,  showing  subperiosteal  and  central  spongy 

bone.    X  '35 

the  ends  of  the  shaft,  where  the  cartilage  still  intervenes  between  the  central 
spongy  bone  and  the  surface,  the  subperiosteal  bone  forms  a  thin  perichon- 
dral shell.  The  two  processes,  central  and  peripheral  bone-formation, 
progress  simultaneously,  so  that  their  products  are  often  seen  in  the  same 
microscopical  field  lying  side  by  side,  separated,  however,  by  a  thin  and 
incomplete  layer  of  calcified  cartilage-matrix,  known  as  the  boundary  line. 
From  their  relations  to  cartilage,  it  is  evident  that  the  subperiosteal  bone 
never  contains  the  remains  of  the  calcified  cartilage,  while  such  enclosures 
are  very  common  within  the  trabeculae  of  the  central  spongy  bone  (Fig.  6i). 
The  conversion  of  the  peripheral  spongy  bone  into  the  typical  compact 
bone  of  the  shaft  involves  the  partial  absorption  of  the  subperiosteal  network 
and  the  secondary  deposit  of  new  osseous  tissue.     The  initial  phase  of  this 


DEVELOPMENT    OF    BONE. 


49 


conversion  is  the  partial  absorption  of  tlie  trabecuUe  by  tlie  osteoclasts  within 
the  primary  marrow.  As  the  result  of  this  process,  the  robust  and  close 
reticulum  of  subperiosteal  bone  is  reduced  to  a  delicate  osseous  framework 
enclosing  enlarged  marrow-channels.  The  latter  are  now  known  as  the 
Haversian  spaces  and,  in  cross-section,  are  round  or  oval.  After  the 
destructive  work  of  the  osteoclasts  has  progressed  to  the  required  extent,  the 
osteoblasts  of  the  marrow-tissue  within  the  Haversian  spaces  begin  the  for- 
mation of  new  bone  on  the  walls  of  the  spaces.  This  process  is  continued 
until,  layer  after  layer,  almost  the  entire  space  is  filled  with  concentric 
lamellae.  The  cavity  remaining  at  the  centre  of  the  former  space  persists  as 
an  Haversian  canal,  while  the  concentrically  disposed  layers  of  secondary 
bone  are  the  lamellae  of  the  Haversian  system,  whose  extent  corresponds  to 
the  form  and  size  of  the  Haversian  space.  The  interstitial  or  ground 
lamellae  of  adult  bone  are  the  remains  of  the  trabeculae  of  the  primary  sub- 
periosteal spongy  bone  and  are,  evidently,  genetically  older  than  the  Haver- 
sian lamellae.  The  outer  surface  of  the  subperiosteal  bone  is  beset  with 
depressions  occupied  by  the  primary  marrow-tissue.  As  these  pits  deepen 
in  consequence  of  the  increasing  thickness  of  the  growing  bone^  they  are 
converted  into  the  nutrient  channels  which  occupy  the  circumferential 
and  ground  lamellae.  They  are,  therefore,  not  surrounded  by  Haversian 
layers  and  correspond  to  the 


Osteogenetic 
tissue 


v'x^-^ 


"  Blood-vessel 


Osteoblasts 


Volkmann  canals,  through 
which  so  many  nutrient  ves- 
sels enter  the  bone. 

Intramembranous 
Bone-Development. — 
The  bones  not  preceded  by 
masses  of  cartilage,  as  those 
constituting  the  vault  of  the 
cranium  and  the  greater  part 
of  the  face,  develop  within 
sheets  of  connective  tissue 
by  a  process  which,  although 
differing  in  its  earliest  details, 
essentially  corresponds  to 
subperiosteal  bone-forma- 
tion. Except  where  develop- 
ing muscle  occurs,  the  early 
roof  of  the  skull  consists  of 
the  integument,  the  dura 
mater  and  an  intervening 
stratum  of  young  connective 
tissue.  The  latter  layer  con- 
tains numerous  embryonal 
cells  and  delicate  bundles  of  fibres.  About  the  middle  of  the  area  corre- 
sponding to  the  later  bone,  some  of  these  fibrous  strands  undergo  calcification 
and  thereby  supply  a  radiating  framework  upon  the  surface  of  which  the 
osteoclasts,  derived  from  the  embryonal  connective  tissue  cells,  arrange 
themselves  and  bring  about  the  deposit  of  bone-matrix.  Delicate  spicules 
of  new  bone  radiate  towards  the  periphery  from  the  ossification  centre 
thus  established.  As  the  trabeculae  increase  in  size  and  number,  they  join 
to  form  a  bony  network,  close  and  robust  at  the  centre  and  wide  meshed 
and  delicate  towards  the  periphery  where  the  osseous  reticulum  fades  into 


Young  bone 


Fig.  62. — Periphery  of  a  developing;  membrane-bone  (parietal), 
showing  trabeculae  covered  with  osteoblasts.     X  95- 


50  NORMAL    HISTOLOGY. 

the  connective  tissue.  The  details  of  this  bone-formation  by  the  osteoblasts 
correspond  to  those  seen  in  other  localities,  including  the  conversion  of 
some  of  the  osteoclasts  into  bone-cells.  With  the  growth  of  the  bony 
tissue  the  network  becomes  more  and  more  compact  until  it  forms  an  osseous 
plate,  which  gradually  expands  towards  the  limits  of  the  future  bone. 
During  this  growth,  the  connective  tissue  covering  the  outer  and  inner  sur- 
faces of  the  plate  assumes  the  character  and  arrangement  of  a  periosteum  and 
from  the  osteogenetic  layer  produces  the  compact  surface  lamellae  which 
enclose  the  intervening  spongy  tissue.  This  arrangement  is  seen  in  the  fully 
developed  bone  (Fig.  48),  where  the  so-called  outer  and  inner  tables  enclose 
the  diploe.  The  development  of  the  superficial  layers  of  the  surface  lamellae 
is,  therefore,  identical  with  that  of  other  subperiosteal  surface  lamellae,  while 
the  production  of  the  diploe  corresponds  with  that  of  the  peripheral  spongy 
bone  in  its  essentials,  even  to  partial  absorption  in  order  to  produce  enlarged 
marrow  spaces.  The  secondary  deposit  of  Haversian  lamellae,  however, 
never  takes  place,  the  conspicuous  systems  of  concentric  layers  being  absent 
in  the  membrane-bones.  Increased  thickness  of  the  membrane-bones  follows 
the  addition  of  new  surface  lamellae  ;  increased  area  results  from  the  marginal 
growth  of  the  enclosed  network  of  bony  trabeculae.  In  the  young  skull  the 
vault-bones  are  separated  by  considerable  tracts  of  connective  tissue,  con- 
spicuous in  the  fontanelles  and  the  evident  sutures.  This  isolation  continues, 
in  principle  at  least,  even  after  the  bones  are  in  close  apposition,  and  ends 
only  with  the  complete  replacement  of  the  intervening  periosteum  by  bone, 
such  bony  union  being  subject  to  great  individual  variations  as  to  time 
and  extent. 

Growth  of  Bones. — Since  new  bone  is  deposited  beneath  the  peri- 
osteum, it  is  evident  that  in  a  long  bone  such  growth  results  in  increased 
diameter  of  the  shaft,  as  well  as  in  increased  thickness  of  the  bony  wall 
between  the  central  medullary  cavity  and  the  surface.  In  order  to  maintain 
the  balance  between  the  longitudinal  growth  of  the  marrow-cavity  (effected 
by  the  destruction  of  the  cartilage  and  the  absorption  of  the  intracartilagi- 
nous  bone)  and  its  lateral  expansion,  removal  of  the  innermost  layers  of 
the  subperiosteal  bone  soon  becomes  necessary.  This  is  effected  by  the 
osteoclasts,  absorption  of  the  older  internal  portion  accompanying  the  depo- 
sition of  new  lamellae  on  the  surface.  By  this  combination  of  destructive 
and  formative  processes,  the  thickness  of  the  cylindrical  wall  of  compact 
substance  of  the  shaft  is  kept  within  the  proper  limits  to  insure  the  necessary 
strength  without  undue  weight.  During  early  growth,  increase  in  the  length 
of  the  bone  is  due  to  the  addition  at  the  ends  of  new  cartilage  formed  by  the 
perichondrium  ;  later,  these  additions  are  supplemented  by  interstitial  growth 
following  multiplication  of  the  cartilage-cells.  On  attaining  full  growth  and 
completed  epiphyseal  ossification  (page  47),  a  portion  of  the  cartilage  per- 
sists as  the  covering  of  the  articular  surfaces.  During  the  development  of 
the  short  bones,  in  which  the  entire  bone  is  made  up  by  a  mass  of  spongy 
substance  enclosed  by  a  shell  of  compact  bone,  no  definite  envelope  of  sub- 
periosteal bone  forms  until  the  cartilage  has  completely  disappeared.  The 
subsequent  growth  and  expansion  of  such  bones  is  accomplished  by  the 
superficial  addition  and  internal  absorption  of  the  subperiosteal  bone  and  the 
accompanying  expansion  of  the  central  spongy  tissue.  In  the  flat  bones,  as 
the  scapula,  the  subperiosteal  production  is  well  advanced  before  the  intra- 
cartilaginous  process  begins.  After  the  cessation  of  peripheral  growth,  the 
osteogenetic  layer  of  the  periosteum  becomes  denser  and  much  less  rich  in 
cells,  although  it  retains  an  intimate  connection  with  the  last  formed  lamellae 


ARTICULATIONS. 


51 


by  means  of  the  processes  which  continue  its  tissue  into  the  vascular  channels 
within  the  bone.  In  addition  to  being  the  most  important  source  of  nutrition, 
on  account  of  its  blood-vessels,  the  periosteum  responds  to  demands  for  the 
production  of  new  bone,  whether  for  renewed  growth  or  for  repair,  and, 
when  occasion  requires,  again  becomes  active  as  the  chief  bone-forming 
tissue,  its  cells  reassuminsf  the  role  of  osteoblasts. 


Capsule 

Synovial  membrane 

Articular  cartilage 

Joint  cavity 

Reflection  of 

synovial  membrane 

Epiphyseal  bone 


THE    ARTICULATIONS. 

Broadly  considered,  the  individual  pieces  composing  the  skeleton  are 
united  by  articulations  of  two  kinds:  (i)  the  continuous  joint  (synarf/irosis), 
in  which  the  union  is  effected  by  uninterrupted  masses  of  tissue  and  the  bones 
have  no,  or  only  very  slight,  play;  and  (2)  the  discontinuous  joint  {diarthro- 
sis),  in  which  the  bones  are  joined  by  tissue  containing  definite  joint-cavities 
and,  therefore,  are  free  to  move  on  each  other. 

Synarthrosis  may  be:  (a)  by  dense  connective  tissue  (sufura),  as  in 
the  immovable  articulations  of  the  skull,  where  the  inter\'ening  periosteum  is 
intimately  connected  with  the  bones  by 
penetrating  processes  composed  of  white 
and  elastic  fibres;  (d)  by  ligamentous  tissue 
(syndesmosis')  arranged  in  dense  fibrous 
bands,  which  stretch  between  the  adjacent 
bones  and  permit  of  slight  mo\'ement,  as 
between  the  lower  ends  of  the  tibia  and 
fibula;  and  (c)  by  cartilaginous  tissue  (syn- 
chondrosis) ,  which  affords  a  rigid  or  flexible 
joint  according  to  the  proportions  of  the 
hyaline  or  fibrous  varieties  of  the  tissue. 
Thus,  where  the  bond  of  union  consists 
exclusi\'ely  of  hyaline  cartilage,  as  between 
the  component  pieces  of  a  young  bone, 
immobility  results;  where  fibrous  cartilage 
predominates,  as  in  the  massive  interverte- 
bral disks,  the  union  provides  great  strength 
and  some  flexibility.      Outside  the  spongy 

substance  or  nucleus  pu/posus,  which  occupies  the  centre  of  the  disk  and  is 
regarded  as  the  modified  remains  of  the  chorda  dorsalis  of  foetal  life,  the  inter- 
vertebral disk  consists  of  interwoven  bundles  of  fibrous  cartilage.  Towards 
the  surface  the  more  typical  cartilaginous  tissue  is  replaced  by  a  peripheral 
layer  resembling  tendon  in  structure. 

Diarthrosis  or  the  true  joint  includes,  as  its  essential  parts,  the  artic- 
ular cartilage  and  the  capsule;  interarticular  and  adaptation  cartilages  and 
syno\'ial  fringes  are  secondary  structures  which  may  or  may  not  be  present. 
The  articular  cartilage  covering  the  surfaces  of  the  bones  in  apposition  is, 
with  few  exceptions,  of  the  h valine  varietv.  Next  the  joint- cavity,  the  car- 
tilage-cells are  usually  flattened  and  arranged  parallel  with  the  free  surface; 
deeper,  the  cells  are  more  spherical  in  form  and  disposed  in  groups,  while  in 
the  layers  still  nearer  the  underlying  bone,  the  cartilage-cells  often  show  a 
characteristic  columnar  arrangement,  in  which  the  rows  of  cells  lie  in  a  gen- 
eral way  perpendicular  to  the  surface  of  the  bone.  The  matrix  immediately 
overlying  the  bone  is  commonly  the  seat  of  more  or  less  marked  calcifica- 
tion, a  zone  of  calcified  matrix  thus  forming  the  union  between  the  cartilage 
and  the  bone. 


63. — Diagram  showing  essential 
pans  of  a  typical  joint. 


52 


NORMAL    HISTOLOGY. 


Where  two  joint-cavities  exist,  separated  by  an  interarticular  cartilage, 
the  development  of  the  partition,  or  meniscus,  seems  to  influence  the  struct- 
ure of  the  cartilage  capping  the  articular  surfaces  of  the  bones.  In  such 
cases,  as  in  the  mandibular,  costo-sternal,  sterno-clavicular,  acromio-clavic- 
ular  and  lower  radio-ulnar  articulations,  fibrous  cartilage  not  only  forms 
the  interarticular  plates,  but  also  contributes  the  covering  of  the  bones. 
Such  instances,  therefore,  are  exceptions  to  the  usual  investment  of  hyaline 
cartilage.  The  adaptation  cartilages,  or  lah^a  glenoidalia,  as  the  glenoid 
and  semilunar  cartilages  in  the  shoulder  and  knee  joint  respectively,  that 
serve  to  deepen  the  cups  in  which  the  humerus  and  the  femur  play,  also 
consist  of  dense  fibrous  tissue  containing  rounded  cartilage-cells. 

The  capsule  surrounding  the  joint-cavity  includes  two  layers  :  the 
outer  fibrous  and  the  inner  synovial.  The  fibrous  layer,  made  up  of  inter- 
lacing bundles  of  dense  fibrous  tissue,  varies  much  in  thickness  in  different 


Free  surface  of, 
articular  cartilage 


Bone. 


Marrow-tissue 


Blood-vessel 


"Synovial  membrane 


Junction  of 
cartilage  and 
synovial  membrane 


Fig.  64. — Section  through  margin  of  joint,  showing  articular  cartilage  and  synovial  membrane. 


joints,  in  the  minute  articulations  between  the  ear-ossicles  being  a  delicate 
membrane,  while  in  the  capsule  of  the  hip-joint  it  reaches  almost  a  centimeter. 

The  synovial  layer,  or  synovial  membrane,  consists  of  loose  connective 
tissue  containing  elastic  fibres  and  more  or  less  extensive  groups  of  fat- 
cells;  next  the  joint-cavity  the  tissue  is  condensed  into  a  narrow  compact 
stratum,  whose  free  or  joint-surface  is  clothed  with  flattened  connective 
tissue  cells.  The  latter  are  plate-like  elements,  irregularly  oval  or  stellate  in 
outline,  and,  where  closely  placed,  form  a  lining  for  the  capsule  that  resembles 
an  imperfect  endothelium.  While  the  fibrous  layer  of  the  capsule  is  often 
carried  for  some  distance  beyond  the  margin  of  the  joint  to  blend  with  the 
periosteum,  the  synovial  membrane  is  reflected  from  the  capsule  to  the 
bones  and  the  articular  cartilage,  extending  over  the  latter  for  a  variable 
distance,  but  thinning  out  and  disappearing  over  the  surfaces  subject  to 
pressure.  The  actual  articulating  surfaces,  therefore,  are  devoid  of  synovial 
membrane,  the  lubricated  cartilages  coming  into  contact  during  the  move- 
ments of  the  bones. 

Within  the  larger  articulations  the  synovial  membrane  is  thrown  into 
uncertain  folds,  which  project  into  the  joint,  enclose  masses  of  adipose  tissue, 
and  are  beset  with  numerous  minute  elevations.  The  latter,  the  synovial 
villi,  are  found  especially  around  the  margin  of  the  articular  surfaces  and, 


MUSCULAR   TISSUE.  53 

in  most  cases,  contain  loops  of  capillary  blood-vessels,  which,  together  with 
the  other  capillaries  within  the  synovial  membrane,  are  important  in  pro- 
ducing the  fluid  within  the  joint.  Although  this  synovial  fluid,  or  synovia, 
consists  almost  wholly  (94  per  cent. )  of  water,  it  is  slightly  viscid  and,  there- 
fore, well  adapted  to  lubricate  the  articulating  cartilages.  In  addition  to 
salts,  proteid  and  mucoid  substances,  the  synovia  contains  oil  drops  and  the 
remains  of  cells  displaced  by  abrasion. 

B/ood-vesscls  and  nerves  are  wanting  within  the  articular  cartilages,  as 
well  as  within  the  interarticular  and  the  adaptation  cartilages.  The  synovial 
membrane,  on  the  contrary,  possesses  numerous  vessels  and  nerves.  The 
larger  blood-vessels  occupy  the  stratum  of  loose  connective  tissue,  the  capil- 
laries penetrating  into  the  innermost  layer  and  the  villi.  The  nerves  include 
vasomotor  and  sensory  fibres,  some  of  the  latter  being  connected  with  special 
endings  (Vater-Pacinian  bodies  and  Krause's  articular  end-bulbs).  Definite 
lymphatics  are  found  within  the  synovial  membrane  immediately  beneath 
the  joint-surface. 

MUSCULAR   TISSUE. 

Although  possessed  to  some  degree  by  all  li\'ing  protoplasm,  contrac- 
tility is  exhibited  characteristically  by  muscular  tissue.  The  latter  is  made  up 
of  greatly  elongated  elements,  which  during  contraction  shorten  in  the  direc- 
tion corresponding  with  their  long  axes  and,  hence,  exert  a  definite  pull  that 
results  in  motion.  In  the  higher  animals  muscular  tissue  occurs  in  two  chief 
kinds,  the  striated  and  the  Jionstriated,  as  distinguished  by  their  histological 
appearances.  The  former  composes  the  muscles  controlled  by  the  will  and 
therefore  is  also  termed  voluntary  muscle ;  the  latter  acts  independently  of 
volition  and  is  spoken  of  as  involuntary  muscle.  The  association  of  striated 
muscle  with  the  will  and,  conversely,  of  the  nonstriated  variety  with  involun- 
tary action  must  be  made,  howe\'er,  with  reservation,  since-  in  some  animals 
voluntary  muscle  is  without  striations.  There  is,  indeed,  reason  to  believe 
that  the  histological  differences  are  not  fundamental,  but  are  correlated  with 
function.  Thus,  the  muscles  of  the  oesophagus  which  in  one  group  of  ani- 
mals are  striated,  in  another  group  may  be  represented  by  nonstriated  tissue; 
further,  much  of  the  voluntary  muscle  of  the  head  may  be  regarded  as  the 
equivalent  of  the  involuntary  muscle  of  the  trunk.  The  nonstriated  or  invol- 
untary muscle  represents  a  less  highly  specialized  type  than  the  striped,  the 
latter  exhibiting  to  a  conspicuous  degree  histological  differentiation.  As  an 
intermediate  group  stands  the  muscular  tissue  composing  the  heart,  since  the 
cardiac  muscle  is  beyond  the  control  of  the  will  although  it  possesses  striated 
fibres.  The  latter  occupy  a  position,  therefore,  between  the  fibre-cells  of  the 
involuntary  muscle  and  the  elongated  striated  fibres  of  the  voluntary  muscle. 

NONSTRIATED   OR   INVOLUNTARY   MUSCLE. 

This  variety  of  muscular  tissue  occurs  in  the  form  of  bundles  and  thin 
sheets  chiefly  within  the  walls  of  the  hollow  viscera  and  of  the  vessels  and, 
although  enjoying  a  wide  distribution  in  the  body,  seldom  forms  considerable 
masses.  Its  distribution  includes:  i.  The  digestive  tract — the  muscularis 
mucosae  from  the  oesophagus  to  the  anus  and  delicate  bundles  within  the 
mucosa;  the  muscular  tunic  from  the  lower  half  of  the  oesophagus  to  the 
anus;  in  the  large  excretory  ducts  of  the  liver,  pancreas,  and  some  salivary 
glands,  as  well  as  in  the  wall  of  the  gall-bladder.  2.  The  respiratory  tract — 
in  the  posterior  wall  of  the  trachea  and  as  encircling  bundles  in  the  walls  of 
the  air-tubes.      3.   The  urinary  tract — in  the  capsule  and  pelvis  of  the  kidney 


54  NORMAL   HISTOLOGY. 

and  in  the  walls  of  the  ureter,  bladder  and  urethra.  4.  The  male  repToduc- 
tive  organs — in  the  epididymis,  vas  deferens,  seminal  vesicles,  prostate  and 
Cowper's  glands,  and  cavernous  and  spongy  bodies  of  penis.  5.  T\\&  female 
reproductive  organs — in  the  oviducts,  uterus  and  vagina;  in  the  broad  and 
round  ligaments;  in  the  erectile  tissue  of  the  external  genital  organs  and  in 
the  nipple.  6.  The  vascnlar  system — in  the  walls  of  the  arteries,  veins  and 
larger  lymphatics;  sometimes  in  the  trabeculae  of  the  larger  lymph-nodes;  in 
the  capsule  and  trabeculae  of  the  spleen.  7.  The  eye — in  the  iris  and  ciliary 
region;  in  the  eyelids.  8.  The  integnment — in  the  sweat  and  some  seba- 
ceous glands:  as  the  minute  erector  muscles  of  the  hair-follicles  and  in  the 
skin  covering  the  scrotum  and  parts  of  the  external  genital  organs. 

Nonstriated,  smooth,  pale,  unstriped  or  involuntary  muscle,  as  it  is 
variously  designated,  consists  of  structural  units  known  as  the  fibre-cells. 
These  are  delicate  spindle,  often  prismatic,  elements  whose  tapering  ends  fit 


Fig.    65.— Involuntary  muscle  from  intestine;  several  isolated  fibre-cells  are  seen  above.     X  200. 

between  the  adjacent  fibre-cells.  They  vary  greatly  in  size,  measuring  from 
50-225  ,a  in  length  and  from  3-8  p.  in  width.  The  fibre-cells  found  in  the 
skin  and  the  blood-vessels  are  short  (15-20  //)  and  broad;  those  in  the  intes- 
tinal wall  are  more  elongated  (215—220  //.)  and  delicate.  The  largest 
elements  are  encountered  in  the  gravid  uterus  where  they  may  attain  a  length 
of  500  II  and  a  width  of  30  /./..  Occasionally  the  cells  are  bifurcated,  espe- 
cially among  the  lower  vertebrates.  Each  fibre-cell  consists  of  protoplasm  in 
which  are  embedded  the  nucleus  and  the  contractile  fibrillce.  The  nucleus, 
usually  described  as  rod-shaped  from  its  elongated  oval  or  cylindrical  form, 
is  placed  frequently  somewhat  eccentrically  with  regard  to  the  long  axis  and 
nearer  one  pole  than  the  other.  It  is  rich  in  chromatin  which  usually 
presents  a  reticular  arrangement.  Influenced  by  contrac- 
\  tion,  the  nuclei  often  exhibit  deviations  from  the  typical 

~*-tj       rod-form.      Paired    centrosomes    have    been    observed 
within  the  cytoplasm  close  to  the  side  of  the  nucleus. 
,^     _^_^,j,      The   contractile  fibrillae,  convincingly  seen  only  within 
W^^^S^^^     the  large  elements  of  the  amphibia,  are  doubly  refract- 
^*^%^E^^         ing  (anisotropic)  threads  within  the  cytoplasm.      They 
Fig.    66.— Bundles    of    lie  at  the  periphery  of  the  fibre-cell,  closely  related  to 
transverse^secUon^^s^how"    the  denser  boundary  zone,  which  forms  the  exterior  of 
ing  the    fibre-cells    cut    ^-^e    fibre-ccll    and    fulfils    the    purpose    of    a    limiting 

crosswise.     X  400-  ,  1  111  1        j    /:     • 

membrane,  or  sarcolemma,  although  no  such  definite 
structure  encloses  the  smooth  muscle-cell  as  in  the  case  of  the  striated  fibre. 
The  individual  fibre-cells  are  held  together  by  an  exceedingly  delicate 
investment  of  connective  tissue  fibres,  both  white  and  elastic,  which  surround 
the  muscle  elements  and  in  cross-sections  appear  as  lines,  formerly  inter- 
preted as  cement-substance,  that  pass  between  and  around  the  fibre-cells. 
Since  the  latter  are  fusiform,  their  transverse  areas,  irregularly  oval  or  polyg- 
onal in  outline,  vary  with  the  plane  of  section,  being  relatively  large  and 
nucleated  when  cut  through  the  middle  of  the  fibre-cell  and  progressively 
smaller  towards  the  ends  (Fig.  66). 


CARDIAC   MUSCLE. 


55 


The  blood-vessels  supplying  involuntary  muscle,  meagre  in  comparison 
with  those  of  the  striped  muscle,   are  guided  in  their  distribution  by  the 


septa  of    connective  tissue. 


I  it  \ 


I 


in  which  the  larger  vessels  run.  These  give 
of?  minute  branches  that  terminate  in 
capillary    networks    which    extend   be- 

I  tween  the  primary  bundles  of  fibre-cells. 

Numerous  lymphatics  likewise  follow  the 


}^ 


*^ 


Fig.  67. — Section  of  uterus,  showing  bundles 
of  involuntary  muscle  cut  in  various  directions. 
X  220. 


Fir,.  68. — Portion  of  injected  intestinal  wall, 
showing  arrangement  of  blood-v'essels  supply- 
ing involuntary  muscle  ;  upper  layer  longitudi- 
nally, lower  transversely  cut.     X  50. 


larger  septa  of  connective  tissue.  The  nerves  supplying  involuntary  muscle 
are  sympathetic  fibres.  The  larger  trunks  form  plexuses,  closely  associated 
with  microscopic  ganglia,  from  which  delicate  twigs  pass  between  the  bun- 
dles of  fibre-cells.  Their  ultimate  relation  with  the  contractile  tissue  is 
described  with  the  Nerve-Endings  (page  86). 


CARDIAC    MUSCLE. 

The  contractile  tissue  constituting  the  greater  bulk  of  the  heart  repre- 
sents a  type  of  muscle  which,  so  far  as  histological  differentiation  is  con- 
cerned, stands  between  the  simpler  smooth  muscle  and  the  highly  complex 
striated  tissue.  The  striking  peculiarity  of  cardiac  muscle,  namely  its  re- 
ticular arrangement,  is  referable  to  embryonic  conditions.  The  mesenchyma, 
from  which  the  heart-muscle  develops,  for  a  time  exists  as  a  protoplasmic 
reticulum  that  contains  irregularly  distributed  nuclei  but  is  without  cell- 
boundaries.  This  tissue  corresponds,  therefore,  to  a  syncytium.  As  the 
syncytial  network  becomes  more  compact,  owing  to  the  increasing  width  of 
its  trabecuke  with  corresponding  diminution  of  the  intervening  spaces,  deli- 
cate contractile  threads,  the  myo-fibrils,  make  their  appearance  within  the 
reticulum  and  extend  lengthwise  through  the  trabeculae,  without  regard  to 
the  limits  of  the  future  cell-areas.  Notwithstanding  the  differentiation  of 
the  greater  part  of  the  syncytium  into  contractile  fibrillae  and  the  conversion 
of  the  spongy  embryonal  tissue  into  the  compact  tissue  of  the  heart-wall, 
evidences  of  the  primary  reticular  arrangement  are  seen  in  the  characteristic 
networks  formed  by  the  adult  cardiac  muscle. 


56 


NORMAL   HISTOLOGY. 


When  examined  in 
the  fibres  (Fig.  69),  the 
anastomosing  trabeculae. 


sections  passing  parallel  to  the  general  course  of 
heart-muscle  exhibits  a  close  irregular  reticulum  of 
The  latter  consist  of  strands  of  striated  muscle  in 
which  lie  oval  nuclei,  surrounded  by  pale  areas 
of  undifferentiated  granular  cytoplasm,  or  sarco- 
plasjji,  devoid  of  striations.  In  cross-sections 
(Fig.  70),  the  contractile  fibrillae  are  seen  to  be 
arranged  in  radiating  groups  which  occupy  the 
periphery  of  the  trabeculse  but  do  not  reach 
inward  as  far  as  the  axis.  The  latter  consists  of 
a  variable  core  of  sarcoplasm,  which  surrounds 
the  nucleus  and  usually  contains  a  small  quantity 
of  pigment  and  fatty  particles.  In  its  general 
histological  details,  cardiac  muscle  agrees  with 
typical  striped  muscle,  the  alternate  light  and 
dark  stripes  depending  upon  similar  variations 


Undifferentiated 
sarcoplasu 


*  *  ■')      Capillary 

■y*^ blood- 

"U*/  vessel 


Fig.  69. — Muscle-fibres  of  human 
heart.     X  375- 


Fig.  70.- 


-Fibres  of  cardiac  muscle  in  transverse  section. 

X  375- 


of  density  along  the  component  fibrillae.  The  probable  significance  of  these 
markings  will  be  considered  under  Striated  Muscle  (page  58)  ;  suffice  it 
here  to  indicate  the  peculiarities  in  which  the  muscular  tissue  of  the 
heart  differs  from  typical  striated  muscle.  Although  invested  by  a  delicate 
sheath,  the  cardiac  fibres  do  not  possess  a  well  defined  sarcolemma.  Their 
longitudinal  striation,  often  very  distinct,  owing  to  the  large  amount  of  sar- 
coplasm between  the  groups  of  fibrillae,  is  interrupted  at  uncertain  intervals 
by  dark  transverse  markings,  the  intercalated  disks,  which,  however,  gener- 
ally are  shorter  than  the  width  of  the  trabecula,  so  that  two  or  three  such 
disks  are  required  to  complete  the  diameter  of  the  strand.  Further,  the 
disks  do  not  lie  on  the  same  plane,  but  at  different  levels.  After  dissociation 
reagents,  such  as  a  solution  of  caustic  potash,  heart-muscle  breaks  up  into 
irregularly  branched  pieces,  the  so-called  Jib?^es.  The  lines  of  fracture  cor- 
respond in  position  with  the  intercalated  disks  and,  consequently,  the  ends 
of  the  isolated  fibres  often  are  not  straight,  but  exhibit  a  series  of  offsets  like 
steps.  The  incessant  contraction  of  the  cardiac  muscle,  necessitated  by  its 
function,  is  reflected  in  its  structure,  the  unusually  large  amount  of  sarco- 
plasm which  it  contains  recalling  a  similar  condition  observed  in  the  "red" 
skeletal  muscles  (page  59),  in  which  a  lower  degree  of  differentiation 
seems  associated  with  the  power  of  enduring  frequently  repeated  contraction. 


STRUCTURE  OF  MUSCLE.  57 


STRIATED    MUSCLE. 

The  striped  muscular  tissue  forms  the  conspicuous  masses  known  as  the 
' '  muscles' '  or  "  flesh' '  attached  to  the  bony  framework  of  the  body.  These 
organs  are  the  active  agents  in  moving  the  passive  levers,  the  bones,  and  in 
producing  the  movements  of  the  animal.  The  structural  unit  of  voluntary 
muscle  is  the  transversely  striated  )niisclc-fibn\  which  represents  a  highly 
specialized  cell.  The  fibres  are  the  contractile  elements  by  whose  shortening 
the  length  of  the  entire  muscle  is  decreased  and  its  force  exerted. 

The  muscle-fibres  are  cylindrical,  or  prismatic  with  rounded  angles, 
in  form  and  vary  from  .01-.  i  mm.  in  diameter.  No  constant  relation  exists 
between  the  thickness  of  the  fibres  and  the  size  of  the  muscle  of  which  they 
are  components,  and,  indeed,  their  diameter  varies  in  the  same  muscle.  The 
length  of  the  muscle-fibres  is  likewise  subject  to  great  variation.  As  a  rule, 
the  hbres  are  of  limited  length,  not  exceeding  from  4-5  cm. ;  in  exceptional 
cases,  as  in  the  sartorius  muscle,  they  may  attain  a  length  of  over  12  cm. 
The  fibres  are  usually  slightly  larger  in  the  middle  than  at  the  ends,  which 
are  more  or  less  pointed,  but  sometimes  blunted  or  club-shaped  or,  rarely, 
branched.  Branched  and  anastomosing  fibres  occur  in  the  lingual,  facial 
and  ocular  muscles. 

Each  muscle-fibre  corresponds  to  an  enormously  elongated  multinu- 
cleated cell  and  consists  of  a  sheath,  or  sarcolemma,  and  the  contained  sar- 
coHS  substance.     The  sarcolemma  forms 

a  complete  investment  of  the  fibre  and  "^  ,.-rr?7r^~^ 

alone  comes  in  contact  with  the  sur- 
rounding connective  tissue  by  which  the 
muscle-fibres  are  attached  to  one  another 
or  to  the  fibrous  structures  upon  which 
they    directly   exert    their    pull.      The 

sarcolemma  is  a  transparent,  homo-  sarcolemma" 

geneous,  elastic  membrane  and  envelops  fig.  yi.-Portion  of  muscie-fihre,  shoNvin? 
the  contained  sarcous  substance  so  |fanci^"'™*  bridging  break  in  sarcous  sub- 
closely  that,  under  ordinary  conditions, 

it  is  almost  or  entirely  invisible.  Being  tougher  than  the  muscle-substance, 
it  often  withstands  teasing  with  needles  while  the  muscle  is  broken ;  where 
such  breaks  occur,  the  muscle-substance  sometimes  contracts  within  the 
sarcolemma,  which  then  becomes  visible  at  the  fractures  as  a  delicate  tubular 
sheath  (Fig.  71).  Sometimes  the  sarcolemma  may  be  seen  projecting 
beyond  the  sarcous  substance,  as  a  coat  sleeve  covers  the  stump  of  an  arm. 

The  sarcous  substance,  ev^erywhere  enclosed  within  the  sarcolemma, 
also  consists  of  two  parts,  the  less  differentiated  and  passive  sarcoplasm  and 
the  highly  specialized  contractile  fibrillcE  in  which  take  place  the  active 
changes  resulting  in  the  contraction  of  the  muscle-fibre.  The  characteristic 
cross-striation,  resolvable  into  alternating  light  and  dark  bands,  that  dis- 
tinguishes the  fibres  of  voluntarv  muscle  depends  upon  the  constitution  and 
arrangement  of  the  contractile  fibrillae.  These  are  threads  of  great  tenuity, 
which  extend  the  entire  length  of  the  muscle-fibre  and  present  series  of  alter- 
nating light  and  dark  segments  that  probably  correspond  to  differences  of 
density.  The  dark  denser  areas  are  doubly  refracting  {anisotropic^;  the  light 
less  dense  ones  are  singly  refracting  {isotropic).  The  cross-striation  of  the 
muscle-fibre  as  a  whole  results  from  the  definite  and  orderly  arrangement  of 
the  fibrillce.      Close  lateral  approximation  of  the  denser  and  deeply  staining 


58 


NORMAL   HISTOLOGY. 


Fig.  72. — Diagrams  illustrating-  struct 
ure  of  striated  muscle-fibre.  A.  usua 
view;  B,  correct  view,  showing  sustentac- 
ular  septa  continued  across  fibre  from 
sarcolemma.  Z,  intermediate  di'^ic  (Zwi- 
schenscheibe);  J,  light  band;  Q,  trans- 
verse disk  (Querscheibe)  ;  M,  median  disk 
(Mittelscheibe)\  S,  sarcolemma.  {After 
M.  Ileidenhain.) 


segments  of  the  fibrillae,  lying  side  by  side  within  the  sarcolemma,  produces 
the  impression  of  the  dark  band;  the  similar  relation  of  the  less  dense  and 

slightly  staining  segments  produces  the  light 
band.  If  it  were  possible  to  isolate  the 
individual  contractile  fibrillae,  each  would 
exhibit  the  details  shown  in  the  accompany- 
ing diagram  (Fig.  72).  The  dark  broad 
tra?isverse  disk  (Q)  of  anisotropic  substance 
is  succeeded  at  each  end  by  the  light  band 
{//)  of  isotropic  substance.  The  light 
band  is  subdivided  by  a  delicate  line,  the, 
intermediate  disk  (Z),  also  known  as 
ICraicse's  memh^ane.  The  sequence  which 
by  repetition  makes  up  the  contractile  fibrilla 
is,  therefore,  Z  +  /-f-^+/+Z  In 
favorable  preparations,  the  transverse  disk 
appears  less  dense  and  lighter  midway  bcT 
tween  its  ends  where  it  is  traversed  by  a 
delicate  line  (vJ/),  the  median  disk  (Hensen) 
or  middle  membrane  (Heidenhain).  The 
striped  muscle  of  certain  insects  exhibits  an 
additional  band,  the  accessory  disk,  subdivid- 
ing the  light  zone,  J.  The  interpretation 
of  these  details,  shown  as  ordinarily  seen 
under  moderately  high  magnification  in 
Fig.  73,  has  been  the  subject  of  vexed  discussion.  This  has  been  particularly 
true  of  the  significance  of  the  intermediate  disk  or  membrane  of  Krause, 
which  is  attached  to  the 
inner  surface  of  the  sarco- 
lemma and  extends  com- 
pletely across  the  muscle- 
fibre.  This  arrangement, 
however,  does  not  imply 
that  the  fibre  is  composed 
of  a  series  of  separate  dis- 
coidal  segments,  but  rather 
that  the  membrane  serves 
to  maintain  in  definite 
order  the  contractile  fibril- 
lae, which,  while  perhaps 
attached  to  the  mem- 
brane, pass  uninterrupted- 
ly through  it. 

The  distribution  of  the 
contractile  fibrillae  with- 
in the  fibre  is  not  uni- 
form, since  the  fibrillae  are 
grouped  into  minute  bun- 
dles, the  muscle-cohanns  or 
sarcostyles.  This  arrange- 
ment is  shown  in  transverse 
sections  of  muscular  tissue 
seen  to  be  made  up  of  stippled  areas  separated  by  clear  lines.      These  areas, 


Fig.  73.- 
the  usual 
X  700- 


-Photograph  of   striated  mammalian  muscle,  showing 
appearance    under    moderately    high    magnification. 


(Fig.    74),   in  which,  the  individual    fibres  are 


STRIATED    MUSCLE. 


59 


"W  V 


Eiidoin\bium- 


known  as   Cohnheirn' s  fields,  represent  the  transversely  cut  groups  of  con- 
tractile   tibrillae,   each  dot  corresponding  to  a  sarcostyle.      The  clear  lines 
indicate    the    distribution   of   the    sarcoplasm ;    in    addition    to    intervening 
between  the  fields  of  Cohnheim,  the  sarco- 
plasm separates  the    groups  of   individual 
tibrilUe,     each    sarcostyle    being    entirely 
surrounded  by  the  less  highly  differentiated 
substance. 

Each  muscle-fibre  corresponds  to  a 
multinucleated  cell.  The  numerous  nuclei 
result  from  division  of  the  nucleus  of  the 
embryonal  cell,  the  myoblast,  and  remain 
embedded  within  the  sarcoplasm  as  the 
mitscle-miclei.  Their  usual  position  in 
mammalian  muscle  is  immediately  beneath 
the  sarcolemma;  in  certain  fibres,  how- 
ever, as  in  the  ' '  red ' '  fibres  of  the  ocular 
and  respiratory  muscles,  the  nuclei  lie 
more  deeply  embedded,  therein  agreeing 
in    position  with    the    nuclei   in    the    muscles    of    many   lower  vertebrates. 

The  individual  muscle-fibres,  each  invested  in  its  sarcolemma,  are  grouped 
into  sxwAX  prima)-}'  bundles,  the  component  fibres  of  which  are  held  together 

by  a  small  amount  of  con 

//^  -\ 


w 


Nuclei  of  ititerfibnllar 
tissue 


Fig.  74. — Muscle-fibres  of  lizard  in  trans- 
verse section,  showing  fields  of  Cohnheim 
and  muscle-nuclei.     X  650. 


^r 


^.^ 


X<^r 


//X 


V- 


nective  tissue,  the  endomys- 
iian.  The  latter  is  continu- 
ous with  the  envelope  of 
the  primary  bundles,  the 
perimysium  (Fig.  75). 
The  primary  bundles  are 
associated  into  uncertain 
groups,  the  secondary  bun- 
dles, which  are  united  and 
surrounded  by  extensions 
and  subdivisions  of  the 
general  connective  tissue 
sheath  of  the  muscle,  the 
epimysiuni.  In  muscles 
possessing  a  fine  grain,  the 
secondary  bundles  corre- 
spond with  K\\q: fasciculi,  but 
in  muscles  of  coarse  text- 
ure each  fasciculus  includes 
a  number  of  secondary 
bundles. 

Although  the  skeletal 
muscles  are  usually  pale 
in  tint  and  contract  ener- 
getically when  stimulated, 
particular  muscles  of  cer- 
tain animals,  as  the  semi- 
tendinosus  and  the  soleus  of  the  rabbit,  exhibit  a  deeper  colbr  and  contract 
more  slowly  and  prolongedly  under  stimulation.  Such  red  muscles,  as  they 
are  called,    are  composed  of    fibres  which    are  thinner  than    common  and 


Peri- (£L 

mysium         if" 


Individual 
muscle- 
fibres 


in' 

P.- 
I 


^/ 


Fig. 


7.S- — Several  primary  muscle-bundles  in  transverse  section, 
showing-  the  arrangement  of  component  fibres.      ■'.  40. 


6o 


NORMAL    HISTOLOGY. 


possess  a  relatively  large  amount  of  sarcoplasm,  with  nuclei  embedded 
not  only  beneath  the  sarcolemma,  but  also  in  deeper  parts  of  the  fibres 
(  Fig.  76).  Although  not  present  in  mammals  generally  in  sufficient  numbers 
to  affect  the  appearance  of  entire  muscles,  the  ' '  red ' '  fibres  occur  probably 
in  all  striated  muscular  tissue  upon  which  devolves  prolonged  and  frequently 
repeated  effort.  Such  fibres,  therefore,  are  present  in  the  heart,  eye-muscles, 
and  the  muscles  of  respiration  and  of  mastication.      Possessing,  as  they  do, 

a  larger  proportion  of  undifferentiated 
cytoplasm,  the  sarcoplasm,  the  red 
fibres  are  better  able  to  withstand 
the  fatigue  of  contraction.      The  pale 


Fig.  76. — Portion  of  the  soleus  muscle  of  the  rabbit 
in  transverse  section.  The  more  coarsely  stippled 
fibres  are  of  "  red  "  muscle  ;  they  also  contain  nuclei 
within  the  sarcous  substance.     X  160. 


Longitudinal 


Transverse 


Fig.  77. — Injected  voluntary  muscle,  showing 
arrangement  of  interfascicular  vessels  and  cap- 
illaries.    X  50- 


fibres,  on  the  contrary,  gain  in    rapidity  of  contraction  at  the  expense  of 
early  exhaustion. 

The  blood-vessels  supplying  striated  muscle  are  very  numerous. 
The  larger  arteries  and  accompanying  veins  enter  the  muscle  along  the 
connective  tissue  septa  and  divide  into  smaller  branches  which  run  between 
the  fasciculi.  These  vessels  give  off  twigs  which  pass  between  the  finer 
bundles  of  fibres  and  ultimately  break  up  into  dense  capillary  networks  that 
surround  the  individual  fibres  with  long  rectangular  meshes.  The  relation  of 
the  blood-vessels  to  cardiac  muscle  is  exceptionally  intimate,  the  capillaries 
not  only  enclosing  the  trabeculse  with  rich  networks,  but  also  lying  in  grooves, 
or  even  in  channels,  surrounded  by  the  muscular  tissue.  The  lymphatics 
are  represented  by  the  clefts  within  the  connective  tissue  around  the  fibres 
and  by  definite  lymph-vessels  which  accompany  the  blood-vessels  in  the 
larger  tracts  of  connective  tissue.  The  nerves  supplying  striated  muscle 
include  both  motor  and  sensory  fibres.  The  former  terminate  in  specialized 
end-arborizations,  the  motor  nerve-endi7igs,  which  lie  beneath  the  sarcolemma 
and  upon  the  sarcous  substance.  The  sensory  fibres  are  connected  with  the 
neuro-muscular  end-organs,  or  viusde-spindles.  The  description  of  both  vari- 
eties of  terminations  will  be  found  under  the  Nerve-Endings  (pages  84,  85). 


DEVELOPMENT   OF    MUSCLE.  6i 

Development  of  Muscular  Tissue. — With  the  exception  of  the 
involuntary  muscle  connected  with  the  sweat-glands  and  the  dilator  fibres  of 
the  iris,  all  of  which  probably  arise  from  the  ectoderm,  muscular  tissue  is  a 
derivative  from  the  middle  germ-layer.  The  voluntary  muscles  composing 
the  skeletal  group,  however,  arise  from  the  modified  mesoderm  that  forms 
the  periphery  of  the  quadrate  areas,  or  so»iitcs,  of  the  early  embryo,  while 
the  tracts  of  involuntary  muscle  and  the  heart-muscle  are  de^'eloped  from 
the  mesenchyma.  The  details  of  their  histogenesis  vary  in  each  case.  Since 
invohintaTy  muscle  is  the  musculature  of  the  viscera,  it  generally  develops 
within  the  walls  of  tubes.  Certain  of  the  mesenchymal  cells  undergo  pro- 
liferation  and   elongation  and   gradually   assume   the   characteristics  of  the 


Mesentery  Mesothelium  of  serous  coat 


Differentiating: 
muscle 


\  OUIIJJ 

connective  tissue 


Epithelium 
lining  gut  tube^'  ^'^•^'S; 


'^^..S^i'SUsi  ^,^-^ 


Fig.   78. — Section   of    developing   intestinal   wall,    showing   differentiation  of   involuntary  muscle  from 

splanchnic  mesoderm.      ■    iSo. 

fusiform  fibre-cell.      Their  primary  loose  arrangement  gives  place  to  com- 
pactness with  reduction  of  the  intervening  connective  tissue. 

The  cardiac  muscle  originates  from  the  nucleated  protoplasmic  retic- 
ulum, the  syncytium,  formed  by  the  mesenchyma  of  the  early  heart-tube. 
Contractile  threads,  the  myo-fibriles,  make  their  appearance  within  the  pro- 
toplasmic trabecuke  and  increase  in  number  by  longitudinal  splitting.  The 
contractile  fibrillae  are  for  a  time  homogeneous,  but  later  undergo  differentia- 
tion in  density  into  light  and  dark  segments,  which  produce  the  general 
transverse  striation  of  light  and  dark  bands.  The  contractile  fibrillae  are  not 
uniformly  distributed  throughout  the  substance  of  the  trabecular,  but  appear 
in  groups  that  condense  into  narrow  wedges,  whose  bases  lie  at  the  periphery 
of  the  fibre,  with  the  thin  edges  towards  the  centre.  The  portion  of  the 
cytoplasm  which  is  not  converted  into  fibrillae  remains  as  an  undifter- 
entiated  sarcoplasm,  filling  the  intervals  between  the  groups  of  contractile 
threads.  After  the  appearance  of  the  intercalated  disks,  the  trabeculae  are 
subdivided  into  irregular  areas,  the  so-called  muscle-fibres,  which  seem  to 
have  only  a  questionable  morphological  significance  as  fundamental  units. 
With  the  progressive  increase  in  the  muscular  substance,  the  intertra- 
becular  spaces  and  the  contained  embryonal  connective  tissue  rapidly 
diminish,  the  former  spongy  tissue  becoming  finally  condensed  into  the 
definitive  heart-muscle. 


62 


NORMAL    HISTOLOGY. 


Neural  canal 


Lateral  plate 
Medial  plate 


The  striated  muscle  fibres  arise  from  the  greatly  elongated  myoblasts  of- 
the  mesodermic  somites.  At  first  spindle  shaped  and  composed  of  granular 
cytoplasm,  the  embryonal  cell  increases  rapidly  in  length,  while  its  nucleus 
_  undergoes     active     proliferation, 

but  for  a  time  is  devoid  of  all 
fibrillae.  These  appear  first  in 
the  periphery  of  the  young  mus- 
cle-cell and  probably  arise  by 
fusion  of  linear  rows  of  granules. 
They  increase  in  number  by  lon- 
gitudinal cleavage  and  are  dis- 
posed in  groups  separated  by 
the  undifferentiated  sarcoplasm. 
During  the  growth,  the  nuclei 
migrate  from  the  interior  to  the 
surface  of  the  fibre,  where,  beneath 
the  sarcolemma  already  formed, 
they  are  found  regularly  arranged 
in  the  completely  differentiated 
fibre,  in  which  relatively  little 
sarcoplasm  remains.  Coincident 
with  the  growth  and  increased 
number  of  the  muscle-fibres,  the 
intervening  embryonal  connective 
tissue  becomes  reduced  to  the 
meagre  endomysium  holding  the 
individual  fibres  together,  while 
that  surrounding  the  primary  bun- 
dles becomes  the  perimysium 
and  the  general  muscle-sheath. 
During  the  growth  of  the  young 
muscle,  actual  increase  in  the 
number  of  fibres  occurs,  but  even  during  childhood,  and,  still  more,  subse- 
quent to  attaining  its  full  size,  enlargement  is  due  chiefly  to  increased  diameter 
of  the  existing  fibres  ow- 
ing to  multiplication  of  the 
fibrillae. 

Attachment. —  The 
attachment  of  the  muscle  fi- 
bres, whether  to  one  anoth- 
er or  to  tendons,  aponeuro- 
ses, periosteum  or  fasciae,  is 
accomplished  by  the  union 
of  the  fibrous  attachments 
with  the  strands  of  connect- 
ive tissue  between  the  fibres 
and  never  directly  with 
the  sarcous  substance.  On 
joining  a  tendon,  the  point- 
ed or  obliquely  ending  fi- 
bres, completely  enclosed  within  the  sarcolemma,  are  received  between  the  ten- 
don-bundles (Fig.  82)  which  fuse  with  the  endomysium.  A  similar  relation 
obtains  where  a  muscle  is  inserted  into  the  periosteum  or  a  fascia;  but  where  the 


Intersegmental 
vessel 


Ectoderm 


Wall  of  neural 
lube 


Neural  canal 


Fig. 


79. — Frontal  section  of  rabbit  embryo,  showing 
myotomes.     X  gS. 


'-<S^sf 


■::itll&>      _-- 


Fig. 


*^ 


So  — Developing    vokintary 
i.nsti  iated. 


muscle  ; 
•'  525- 


the    fibres    are    still 


TENDON-SHEATHS.  63 

muscle  is  attached  to  the  skin,  the  radiating  muscle-fibres  are  continued  by 
tendon-bundles  of  connective  tissue  rich  in  elastic  fibres,  an  arranj^ement  well 
adapted  to  distribute  evenly  the  pull  of  the  muscle  upon  the  integument. 


Fig.  Si. — Developing  inuscle-fibres  iti  which  the  striation  is  just  appearing.     X  375. 

The  aponeuroses  correspond  in  structure  closely  with  the  tendons, 
being  composed  of  parallel  bundles  of  dense  fibrous  tissue  which  are  arranged 
to  form  membranous  structures. 

The  fasciae  consist  of  feltworks  of  bundles  of  white  fibrous  tissue  with 
a  variable,  usually  considerable,   proportion  of  elastic  fibres.      Where  very 
dense,  as  in  the  fascia  lata,  they  somewhat  resemble  tendon-tissue,  the  com- 
ponent bundles  of  fibro-elastic  tissue  being  com- 
pactly disposed,  although  interwoven  and  lacking 
in  uniform  placing,  and  containing  little  fat.      The  / 

superficial    fasciae,   on  the  other  hand,   consist  of  -        i  '    ' 

loosely  felted  fibro-elastic  strands  and  ordinarily  ^ '  ' 

support  considerable,  at  times  inordinate,  masses  *    /    *     |    * 

of  adipose  tissue.      Where  the  fascia  serves  as  a  /    /   ■  t 

muscle-sheath,  it  contains  little  fat  but  an  unusual  /    ' 

number  of  elastic  fibres,  by  virtue  of  which  it 
accommodates  itself  to  the  changing  form  of  the  I  ' 

enclosed  muscular  tissue. 

Tendon-sheaths. — Where  tendons  play  in  ^ 

bony  grooves,  or  where  it  is  necessary  to  overcome 
some  tendency  to  displacement,   they  are  bound  w    ,       ,<  / 

down  and  held  in  place  by  bands  of  dense  fibrous 
tissue,  which  either  convert  the  grooves  into  canals 
or  form  tubular  investments  that  enclose  the  ten- 
dons, although  allowing  free  longitudinal  move- 
ment. These  connective  tissue  envelopes  constitute 
the  tendon-sheaths  and  may  surround  more  than 
one  tendon.  Each  sheath  consists  of  an  outer 
fibrous  tunic  (vagina  fibrosa)  composed  of  dense      ,^'9-  82.— Section  of  te'idon 

-'  ,        .       ,  ^       ,*  .      ■'  K  .        .  .  showiner  the  termination  of   the 

nbro-elastic  bundles,   contmuous  with  the  perios-     muscie-nbres.    x  200. 
teum  at  the  margins  of  the  groove,  and  an  inner 

synovial  tunic  (vagina  mucosa),  which  lines  the  deeper  surface  of  the 
fibrous  sheath  and  at  the  ends,  or  next  the  bone,  is  reflected,  onto  the 
tendon.  The  latter,  therefore,  is  more  or  less  completely  surrounded  by 
a  double-walled  cylinder,  whose  cavity  is  filled  with  a  fluid  serving  to 
diminish  friction  during  the  play  of  the  tendon.  The  synovial  tunic 
resembles  the  lining  of  joint-cavities,  being  clothed  with  an  imperfect  layer 
of  endothelial  plates.  In  the  larger  sheaths,  minute  vascular  projections 
recall  the  synovial  villi. 


64 


NORMAL    HISTOLOGY. 


Bursae. — In  situations  in  which  a  muscle  or  tendon  moves  over  a  bony- 
prominence,  or  in  which  two  tendons  glide  upon  each  other,  the  intervening 
areolar  tissue  contains  a  space  filled  with  fluid,  termed  a  bursa.  Such  bursae, 
whose  evident  purpose  is  to  diminish  friction,  are  abundant  in  connection 
with  the  limb-muscles  and  in  the  vicinity  of  the  large  articulations  may  com- 
municate with,  the  synovial  cavities.  Conspicuous  examples  of  such  relation 
are  the  subscapular  bursa,  which  opens  into  the  shoulder-joint,  and  the  supra- 
patellar bursa,  which  communicates  with  the  cavity  of  the  knee-joint.  Sub- 
aitaneous  biirsce  are  also  developed  in  the  areolar  tissue  separating  the  super- 
ficial and  deep  fasciae  in  situations  in  which  the  skin  glides  over  a  bone  and 
is  subject  to  pressure,  as  over  the  tip  of  the  elbow.  The  immediate  lining 
of  the  bursal  sac  consists  of  an  incomplete  endothelial  investment,  while  the 
wall  of  the  sac  itself  is  composed  of  dense  fibro-elastic  tissue. 


NERVOUS  TISSUE. 

The  nervous  system — the  complex  apparatus  by  which  the  organism 
is  brought  into  relation  with  its  surroundings  and  by  which  its  various  parts 
are  united  into  one  coordinated  whole — consists  of  structural  units,  the 
neurones,  held  together  by  a  special  sustaining  tissue,  the  neiiroglia,  assisted 
by  ingrowths  of  the  surrounding  connective  tissue.  The  neurone,  the 
morphological  unit  of  the  nervous  system,  includes  a  nucleated  protoplasmic 
accumulation,  the  cell-body,  and  the  processes.  The  former,  usually  spoken 
of  as  the  nerve-cell,  presides  over  the  nutrition  of  the  neurone  and,  in  many 
cases,  is  the  seat  of  the  subtle  changes  giving  rise  to  nervous  impulse.  The 
processes  originate  as  outgrowths  from  the  cell-body  during  development  and 
provide  the  conduction  paths  along  which  impulses  are  conveyed.  They 
are  very  A^ariable  in  length,  some  extending  only  a  fraction  of  a  millimeter 

beyond  the  cell-body,  while  others  continue 
many  centimeters  to  distant  parts  of  the  body. 
The  longer  processes  usually  acquire  protect- 
ing sheaths  and  are  known  as  nerve-fibres. 
These  are  associated  into  bundles  and  consti- 
tute the  nerves  that  pass  to  the  muscles  and 
various  other  organs. 

Reduced  to  its  essential  parts,  the  nervous 
system  consists  of  two  units.  The  one,  the 
sensory  neurone,  takes  up  the  stimulus  received 
upon  the  skin  or  other  sensory  surface  and, 
by  means  of  its  process  (nerve-fibre),  conveys 
such  impulse  from  the  periphery  towards  the 
aggregations  of  nerve-cells  that  lie  in  the 
vicinity  of  the  body-axis.  Functionally,  such 
a  path  consists  of  an  afferent  fibre.  The 
imJDulses  thus  carried  are  transferred  to  the 
second  unit,  the  motor  neurone,  which,  in 
response,  sends  out  the  impulse  originating  within  the  cell-body  (nerve-cell) 
along  the  process  known  as  the  efferent  fibre  to  the  muscle-fibre  and  causes 
contraction.  The  assumed  simple  relations  of  the  foregoing  nervous  appa- 
ratus are,  in  fact,  superseded  by  much  greater  complexity  in  consequence 
of  the  introduction  of  additional  units,  by  which  the  sensory  impulses  are 
distributed  to  nerve-cells  situated  not  only  in  the  immediate  vicinity  of  the 
reception-cell,  but  at  different  and  often  distant  levels. 


Fig.  83. — Diagram  showing  funda- 
mental units  of  nervous  system.  A, 
sensory  neurone,  conducting  afferent 
impulses  by  its  process  (a)  from  periph- 
ery {S)  ;  B,  motor  neurone  sending 
efferent  impulses  by  its  process  (e)  to 
muscle. 


THE   NEURONES. 


65 


Dendrites 


During  the  evolution  of  the  nervous  system  from  the  simpler  type,  the 
cell-bodies  of  the  neurones  forsake  their  primary  superficial  position  and 
recede  from  the  periphery.  This  recession  is  expressed  in  the  axial  accumu- 
lation of  the  cell-bodies  either  within  the  wall,  or  in  the  immediate  vicinity 
of  the  neural  tube  (brain  and  spinal  cord),  to  or  from  which  the  processes 
pass.  The  nervous  system  is  often  divided,  therefore,  into  a  central  and  a 
peripheral  portion.  The  former,  also  known  as  the  cerebrospinal  axis, 
includes  the  brain  and  spinal  cord  and  contains 
the  chief  axial  collections  of  nerve-cells.  The 
peripheral  nervous  system,  on  the  contrary,  con- 
tains the  nerve-cells  of  the  sensory  ganglia  and  is 
composed  principally  of  the  nerve-fibres  that  pass 
to  and  from  the  end-organs.  Intimately  associ- 
ated with  and  in  fact  a  part  of  the  peripheral 
nerv^ous  system,  but  at  the  same  time  possessing 
a  certain  degree  of  independence,  stands  the 
sytnpathetic  system,  which  provides  for  the  inner- 
vation of  the  involuntary  muscle  and  glandular 
tissue  throughout  the  body  and  the  muscle  of 
the  heart. 

The  Neurones. — As  before  stated,  the  neu- 
rones consist  of  the  cell-body  (nerve-cell)  and 
the  processes.  The  latter,  as  seen  in  the  case 
of  a  typical  motor  neurone  extending  from  the 
spinal  cord  to  a  muscle,  are  of  two  kinds  :  (<2) 
the  branched  protoplasmic  extensions,  the  den- 
drites, which  are  usually  multiple  and  form 
elaborate  arborescent  ramifications  that  establish 
relations  with  other  neurones,  and  (b)  the  single 
unbranched  axone  (neuraxis,  neurite)  that  ordi- 
narily is  prolonged  to  form  the  core  or  axis- 
cylinder  of  a  nerve-fibre,  and,  hence,  is  also 
often  termed  the  nerve-  or  axis-cylinder  process. 
Although  ' '  chained  ' '  together  as  the  links  that 
form  the  various  paths  along  which  impulses 
are  conveyed,  the  neurones  are  seldom,  if  ever 
primarily,  actually  united  to  one  another,  but  only 
intimately  related.  Their  processes,  although  in 
close  contact,  are  not  directly  continuous,  contigu- 
ity and  not  continuity  being  the  ordinary  relation. 

The  dendrites  are  usually  uneven  in  contour 
and  robust  as  they  leave  the  cell-body  (nerve-cell),  but  rapidly  become 
thinner  in  consequence  of  their  repeated  division,  until  they  are  reduced  to 
delicate  threads  that  constitute  the  terminal  arborizations,  the  telodendria, 
formed  by  the  end-branches.  The  latter  are  beset  with  minute  enlargements, 
or  varicosities,  and  finally  end  in  terminal  bead-like  thickenings.  The 
axone,  slender  and  smooth  and  of  uniform  thickness,  is  much  less  con- 
spicuous than  the  dendrites.  It  may  be  short  and  extend  only  to  nearby 
cells,  or  it  may  be  of  great  length  and  connect  distant  parts  that  lie  either 
wholly  within  the  cerebro-spinal  axis  (as  from  the  brain-cortex  to  the  lower 
part  of  the  spinal  cord)  or  extend  beyond  (as  from  the  lower  end  of  the 
spinal  cord  to  the  plantar  muscles  of  the  foot).  On  reaching  its  destination, 
the  axone  terminates  in  an  end-arborization  or  telodendrion,  in   a  manner 


Fig. 


Telodendrion 

4. — Diagram    of    typical 
neurone. 


66 


NORMAL   HISTOLOGY. 


Dendrites 


Arborization 

of  axone 


Fig.  85. — Diagram  of  nerve-cell  of 
type  II,  in  which  axone  is  not-  pro- 
longed as  nerve-fibre. 


similar  to  the  dendrites.  Neurones  are  divided  according  to  the  distribution 
of  their  axones  into  two  classes.  In  those  of  the  first  class,  known  as  type  I 
cells,  the  axone  is  continued  as  a  nerve-fibre  and  is,  therefore,  relatively  long. 

Soon  after  leaving  the  cell-body  (nerve-cell), 
such  axones  give  off  delicate  lateral  processes, 
the  collaterals,  which,  after  a  longer  or  shorter 
course,  break  up  into  arborizations  that  end  in 
relation  with  other  and  often  remote  neurones. 
The  neurones  composing  the  second  and  much 
less  frequent  class,  type  II  cells,  possess  short 
axones  that  are  not  continued  as  nerve-fibres,  but 
almost  immediately  break  up  into  complex  end- 
arborizations  or  neuropodia,  limited  to  the  gray 
matter. 

The  nerve-cells,  as  the  cell-bodies  of  the 
neurones  commonly  are  called,  are  in  general 
relatively  large  elements,  those  in  the  anterior 
horns  of  the  spinal  cord  measuring  from  70-150 
/jt.  They  possess  a  large  spherical  nucleus,  poor 
in  chromatin  but  usually  provided  with  a  con- 
spicuous nucleolus.  Their  cytoplasm  varies  in 
appearance  with  the  method  of  fixation  and 
staining  to  such  an  extent,  that  much  uncertainty 
exists  as  to  the  relation  of  many  described  details 
to  the  actual  structure  of  the  cells.  It  is  probable, 
however,  that  the  cell-body  of  the  neurone  consists  of  a  ground- substance, 
homogeneous  or  finely  granular,  in  which  delicate  fibrillcs  and  masses  of 
chromatophilic granules  are  embedded;  in  addi- 
tion a  variable  amount  of  brown  or  blackish 
pigmerd  is  usually  present  in  the  vicinity  of 
the  nucleus.  The  fibrillae,  which  are  continued 
into  all  the  processes  as  far  as  the  terminal 
arborizations,  form  a  dense  network  surround- 
ing the  nucleus  that  is  enclosed  by  a  superficial 
one.  After  special  staining  with  methylene 
blue,  the  chromatophilic  granules  appear  deeply 
colored  and  grouped  in  variable  masses,  known 
as  Nissl  bodies,  which  occupy  the  interstices  of 
the  fibrillar  reticulum.  Collectively  the  gran- 
ules of  ' '  stainable  substance  ' '  constitute  the 
tigroid  substance  and  are  most  conspicuous  in 
the  neighborhood  of  the  nucleus  and  least  so 
at  the  periphery  of  the  cell.  They  are  continued 
into  the  dendrites  as  elongated  flakes  that  finally 
resolve  into  scattered  granules  along  the  proc- 
esses. The  axone,  on  the  contrary,  does  not 
contain  Nissl  bodies,  and  usually  joins  the  cell- 
body  at  an  area  free  from  the  stainable  sub- 
stance, the  process  usually  arising  from  a  slight 
elevation  known  as  the  implantation  cone. 
Exceptionally,  the  axone  arises  from  one  of  the  dendrites,  either  at  its 
base  or  at  some  distance  from  the  cell-body.  Owing  to  the  size  of  the  cells, 
little  more  than  the  stumps  of  the  processes  are  ordinarily  seen  in  sections. 


Fig.  86. — Semidiagratnmatic  repre- 
sentation of  structure  of  neurone;  a, 
axotie. 


THE   NEURONES. 


67 


Every  neurone  possesses  at  least  one  process,  which  is  then  an  axone, 
although  usually  provided  with  both  dendrites  and  axone.  Very  rarely 
more  than  one  axone  is  present.  Depending  upon  the  number  of  their  proc- 
esses, neurones  are  described 
as  unipolar,  bipolar  or  multi- 
polar. The  unipolar  neu- 
rones occur  only  among  the 
lower  vertebrates,  the  apparent 
examples  seen  in  the  familiar 
cells  composing  the  spinal  and 
other  ganglia  connected  with 
sensory  nerve-fibres  resulting 
from  secondary  changes.  Pri- 
marily, such  neurones  possess 
an  axone  and  a  dendrite,  which 
pass  from  the  opposite  ends 
of  the  3''Oung  oval  cell.  Dur- 
ing development,  however,  the 
unilateral  growth  of  the  cell- 
body  brings  about  the  gradual 
approximation  of  the  two  proc- 
esses until  they  fuse  in  the 
single  extension  into  which  the 
flask-shaped  cell  is  prolonged. 
Examples  of  bipolar 
neurones,  in  which  the  den- 
drite and  axone  pass  from 
opposite  sides  of  the  cell-body, 
are  found  in  the  retina  and  the 
ganglia  connected  with  the 
acoustic  nerve.  An  interesting  modification  of  bipolar  neurones  is  presented 
by  the  olfactory  cells,  whose  dendrites  are  represented  by  the  short  micro- 
scopic processes  embedded  with- 
in the  nasal  mucous  membrane, 
whilst  the  axones  are  prolonged 
as  the  fibres  of  the  olfactory 
nerves. 

The  cell-bodies  of  the  multi- 
polar neurones,  which  possess 
one  axone  and  several  dendrites, 
vary  in  form.  Some,  as  those 
within  the  sympathetic  ganglia, 
are  approximately  spherical  and 
of  moderate  size,  with  short 
delicate  dendrites;  many  are  of 
large  size  and  irregularly  stellate 
form,  the  dendrites  passing  out 
in  all  directions,  as  seen  in  the 
conspicuous  motor  neurones  with- 
in the  anterior  cornua  of  the 
spinal  cord;  others  possess  a  regular  and  characteristic  oudine,  as  the 
flask-shaped  cells  of  Purkinje  within  the  cerebellum  or  the  pyramidal  cells 
of   the  cerebral  cortex.      Certain  multipolar    neurones  within    the    cerebral 


Fig.  87. — Nerve-cells  of  human  spinal  cord  stained  to  show 
Nissl  bodies;  Z),  dendrites  ;  ^,  axones;  C,  implantation  cone ; 
N,  nucleus  ;  M,  nucleolus.    X  400. 


Fig.  88. — Diagram  showing 
tratistormation  of  young  bi- 
polar sensory  neurone  into 
one  of  unipolar  type. 


Fig.  89. — Bipolar  neu- 
rones ;  a,  from  olfactory 
mucous  membrane — the 
dendrite  is  above ;  b, 
from    retina.      {Cajal.) 


68 


NORMAL    HISTOLOGY. 


cortex,  and  especially  those  constituting  the  chief 
layer   of   the    cerebellum,    are    distinguished    by 
cell-bodies    and    the   peculiar   ramifications    and 
their  dendrites.      Within    the  cere- 
found  examples  of  multipolar  neu- 
axones  almost  immediately  undergo 
gray  matter  to  which  they  are  con- 
The    Nerve-Fibres. —  From 
above,  it  is  evident  that  nerve-fibres 


components  of  the  granule 
the  small  size  of  their 
claw-like  telodendria  of 
bellar  cortex  are  also 
rones  of  type  II,  whose 
branching  within  the 
fined. 

what    has    been    said 
are  not  independent  ele- 


FlG.  90.- 


-Multipolar  nerve  cells  of  various  forms  ;  A,  from  spinal  cord  ;  £,  from  cerebral  cortex  ;  C,  from 
cerebellar  cortex  ;  a,  axone ;  c,  implantation  cone. 


Axis-cylinders 


ments,  but  only  the  processes  of  neurones — either  the  axones  that  are  pro- 
longed as  fibres,  or  the  dendrites  of  neurones  situated  within  the  spinal  and 
other  sensory  ganglia.  Although  neurones  exist  which  are  not  continued 
as  nerve-fibres,  the  converse  is  not  true,  since  nerve-fibres  are  always 
connected  with  neurones. 

The  fundamental  part  of  every  nerve-fibre  is  the  central  cord,  known  as 
the  axis-cylinder,  which  is  composed  of  delicate  threads,  the  axis-Jibrillce, 

prolonged  from  the  nerve-cell  and  embedded 
within  a  semifluid  interfibrillar  substance,  the 
neuroplasin.  In  the  case  of  the  typical  fibres, 
such  as  form  the  chief  constituents  of  the  periph- 
eral nerves,  the  axis-cylinder  is  surrounded  by 
a  relatively  thick  coat,  known  as  the  medullary 
sheath,  outside  of  which  lies  a  thin  structureless 
envelope,  the  nezirilemma  or  sheath  of  Schwann. 
These  coverings,  however,  do  not  invest  the 
entire  nerve.  Thus,  for  a  short  distance  after 
leaving  the  nerve-cell;  the  axis-cylinder  is  with- 
out covering ;  soon  it  becomes  surrounded  by 
the  medullary  sheath,  and  then,  if  it  be  a 
peripheral  nerve,  acquires  the  outer  envelope, 
the  neurilemma.  In  the  case  of  the  nerve- 
fibres  that  course  within  the  brain  and  spinal 
cord,  the  fibre  is  devoid  of  the  neurilemma, 
although  it  may  possess  the  medullary  coat. 

The  medullary  sheath   consists  of   two    parts,    a    delicate    reticular 
framewoi'k  and  a  fatty  substance,  the  myelin,  that  fills  the  meshes  of  the 


Neurilemma 


.  FiG.gi. — Med ullated  nerve-fibres. 
as  seen  in  longitudinal  section  of 
spinal  nerve.     X  375. 


NERVE-FIBRES. 


69 


Neurilemma 
Axis-cylinder 


Fig.    92.  —  MeduUated    nerve-fibres 
transverse  section.    X  385. 


supporting  reticulum.  The  latter,  arranged  for  the  most  part  as  connected 
membranous  lamellae,  resists  pancreatic  digestion  and  fat-dissolving  reagents 
and  is  composed  of  a  substance  named  iieiirokeratin.  The  reactions  ex- 
hibited by  myelin  indicate  its  fatty  nature, 
this  substance  existing  during  life  perhaps 
in  the  form  of  an  extremely  fine  emulsion 
supported  by  the  framework.  When  fresh, 
myelin  appears  clear  and  highly  refracting 
and  confers  upon  the  nerve-fibres  which  it 
covers,  "medullated  fibres"  as  they  are 
called,  their  characteristic  whitish  color.  It  is 
prone  to  post-mortem  changes,  so  that  after 
death  it  loses  its  former  uniformity  and  pre- 
sents irregular  contractions  and  collections,  or  extrudes  in  irregular  globules 
at  the  ends  of  the  broken  fibres.  The  medullary  sheath  is  not  uniformly 
continuous,  but  is  almost  completely  interrupted  at  regular  intervals  marked 
by  annular  constrictions.  These  constrictions,  the  nodes  of  Raiivier,  corre- 
spond to  very  narrow  zones  at  which  the  medullary  sheath  is  practically 
wanting  and  the  neurilemma  dips  in  and  comes  into  close  relation  with  the 

axis-cylinder.     The  medullary  sheath,  how- 
B  ever,  does  not  suffer  complete  suppression 

at  the  nodes,  but  is  represented  by  a  part 
of  its  framework  which  traverses  the  con- 
strictions. The  latter  occur  at  regular 
intervals  along  the  fibre  which  they  thus 
divide  into  a  series  of  mternodal  segments. 
In  a  general  way,  the  segments  are  longer 
in  large  fibres  (about  i  mm.),  and  shorter 
in  those  of  small  diameter,  in  which  they 
are  reduced  to  .  i  mm.  or  less  in  length.  The 
axis-cylinder  passes  uninterruptedly  across 
the  nodes  and  is  continuous  from  its  origin 
in  the  nerve-cell  to  its  ending  in  the  terminal 
arborization  (telodendrion).  The  neuri- 
lemma also  suffers  no  break  at  the  nodes, 
but  continues  from  one  segment  to  the 
other.  After  treatment  with  osmic  acid, 
the  medullary  sheath  frequently  is  broken 
by  clear  narrow  clefts  that  extend  obliquely 
from  the  neurilemma  towards  the  axis- 
cylinder,  and  thus  subdivide  each  internodal 
segment  into  a  number  of  smaller  tracts, 
known  as  the  Schmidt- Layitejnnann  seg- 
vients.  The  significance  of  this  subdivision 
is  uncertain,  the  details  being  regarded 
by  some  as  artefacts.  Within  each  inter- 
nodal segment,  beneath  the  neurilemma,  lies  a  small  cell,  the  neurilemma 
cell,  which  comprises  an  elongated  oval  nucleus  surrounded  by  a  meagre 
amount  of  cytoplasm.  These  cells  represent  the  remains  of  the  mesodermic 
elements  {sheath  cells')  that  were  active  during  the  growth  of  the  nerve-fibre 
in  providing  its  envelope. 

According  to  the  presence  or  absence  of  the  medullary  coat  throughout 
the  greater  part  of  their  course,  nerve-fibres  are  designated  as  medullated  or 


Node  of  Ranvier 


Fig.  93. — Medullated  nerve-fibres  after 
treatment  with  osmic  acid  ;  A,  fibre  show- 
ing reticulum  within  medullary  coat ;  By 
one  showing:  same  coat  divided  into  seg- 
ments.    ,<  500. 


70  NORMAL   HISTOLOGY. 

nonmedullated.  The  medullated  fibres  constitute  the  majority  of  those 
making  up  the  peripheral  nerves  and  the  fibre-tracts  within  the  cerebro- 
spinal axis.  The  fibres  within  the  latter,  however,  while  medullated  are 
without  a  neurilemma.  The  nonmedullated  fibres,  on  the  other  hand,  are 
chiefly  prolongations  (axones)  from  the  ganglion-cells  of  the  sympathetic 
system,  although  in  the  case  of  the  olfactory  nerves  the  fibres  are  without 
a  neurilemma.  The  distinction  between  these  two  classes  of  nerve- 
fibres,  however,  is  relative  rather  than  absolute,  since  every  medullated 
nerve-fibre  becomes  nonmedullated  not  only  at  its 
origin  from  the  cell,  but  also  before  making  its 
terminal  arborization,  central  or  peripheral.  Medul- 
lated nerve-fibres  vary  from  1-20  p.  in  thickness. 
According  to  their  diameter,  they  are  grouped  ^sfine 
(1-4  /Jt),  medmm  (5-9  //.)  and  coarse  (10-20  /^.)- 
In  a  general  way  it  may  be  said  that  the  thicker 
fibres  are  the  longer  and  are  the  processes  of  large 
nerve-cells;  conversely,  the  finer  fibres  are  shorter 
and  belong  to  small  cells.  Although  subject  to 
many  exceptions,  the  efferent  (motor)  fibres  are 
usually  the  thicker,  and  the  afferent  (sensory)  the 
thinner. 

Since   there   are   many  more  peripheral  nerve- 
fibres  than  nerve-cells,  it  is  evident  that  the  former 
must    undergo    division   along    their   course.      Such 
.  „  ,  ,     doubling  occurs  always  at  a  point  corresponding  to  a 

Fig.   94.  —  NoiimeduUated  ,rS-.  •  •  \  •  •  11  j. 

nerve  fibres  in  longitudinal  nodc  of  Rauvier,  ncvcr  withm  an  mternodal  segment, 
secuon  of  splenic  nerve,  ^j^^  ghcaths  being  Continued  on  the  resulting -fibres. 
On  approaching  their  peripheral  termination,  the 
branching  becomes  more  frequent  and  the  medullary  sheath  thinner  until  it. 
ceases,  after  which  the  axis-cylinder  continues  covered  by  only  the  attenuated 
neurilemma.  The  latter,  now  reduced  to  an  extremely  delicate  covering  be- 
set with  occasional  nuclei,  sooner  or  later  disappears,  the  naked  axis-cylinder 
alone  being  thence  prolonged  to  end  finally  in  the  varicose  threads  of  the  telo- 
dendrion.  The  nonmedullated  nerves  proper,  also  termed  thepa/e fibres 
or  ^bi^es  of  Remak,  include  those  that  are  devoid  of  the  myelin  sheath 
throughout  their  course.  Such  fibres  are  chiefly  the  axones  of  sympathetic 
neurones.  They  are  often  2  11  or  less  in  diameter  and  consist  of  only  the 
axis-cylinder  and  the  neurilemma,  the  latter  being  thin  and  delicate.  The 
pale  fibres,  like  others,  end  in  terminal  arborizations  (telodendria)  composed 
of  naked  axis-cylinders. 

Neuroglia. — Theneurones  (nerve-cells  and  nerve-fibres)  within  the  brain 
and  spinal  cord  are  everywhere  held  together  by  a  special  supporting  tissue 
known  as  neuroglia.  The  latter  is  derived  primarily  from  the  invaginated 
ectoderm  which  forms  the  wall  of  the  neural  tube,  certain  elements,  the  spongio- 
blasts, being  concerned  in  the  production  of  the  neuroglia,  while  others,  the 
neuroblasts,  give  rise  to  the  neurones.  For  a  time  the  supporting  tissue  is 
represented  by  greatly  elongated  radially  disposed  fibre-cells  that  often  ex- 
tend the  entire  thickness  of  the  wall  of  the  neural  tube.  Later,  the  neurog- 
liar  elements  become  differentiated  into:  (a)  those  bordering  the  lumen  of 
the  canal,  where  they  are  partially  retained  as  the  ependymal^  cells;  and  {b) 
those  which  early  migrate  to  more  peripheral  positions  and  give  riseto  st&\- 
hXft  glioge7ietic  cells  that  are  converted  into  spider-like  elements  (Fig.  95). 
In  chrome-silver  preparations  these  appear  as  irregular  triangular  or  quadri- 


NEUROGLIA. 


71 


lateral  cells  from  whose  angles  extend  the  numerous  delicate  Jibrillce  that 
later  become  the  chief  constituents  of  the  neuroglia.  So  long  as  neuroglia 
is  being  produced,  the  gliogenetic  cells  are  present  and  concerned  in  the 
production  of  additional  fibrillse,  their  cytoplasm  becoming  progressively 
reduced  until  in  their  final  condition  of  the  small  glia  cells,  httle  more  than 
the  nucleus  remains.  During  these  changes  very  many  fibrillae  lose  their 
connection  with  the  cells  and,  in  conjunction  with  the  glia  threads  still 
attached,  form  an  intricate  interlacement  in 
which  the  neuroglia  cells,  now  greatly  re- 
duced and  for  the  most  part  devoid  of  proc- 
esses, lie  scattered  at  uncertain  intervals. 

The  mature  neuroglia  everywhere 
consists  of  essentially  the  same  tissues, 
the  differences  noted  in  certain  localities 
depending  largelv  upon  variations    in  its 


Fig. 


95. — Young    neuroglia    cells ;    astrocytes 
from  brain  of  child.     X  300. 


Fig.  96. — Ependyinal  cells  and  adjacent 
neuroglia  surrounding  central  canal  of 
spinal  cord  of  cat.    X  75.    (Rubaschkin.) 


compactness.  Its  chief  constituent  is  the  intricate  feltwork  of  glia-fibres 
which  are  usually  free  but  to  some  extent  connected  with  the  glia-cells. 
Where,  however,  the  neuroglia  borders  the  brain-ventricles  and  the  central 
canal  of  the  spinal  cord  it  presents  special  features.  In  these  situations  it 
forms  the  ependyma,  which  appears  as  a  single-layered  epithelial  lining. 
Within  the  cord,  the  cells  are  pyramidal,  their  bases  looking  towards  the  lumen 
of  the  tube  and  their  apices  towards  the  nervous  tissue.  At  least  during  the 
earlier  years  in  man,  and  throughout  life  in  many  lower  animals,  the  free 
surfaces  of  the  cells  are  beset  with  hair-like  processes  resembling  the  cilia  of 
epithelial  cells.  The  pointed  distal  ends  of  the  ependymal  cells  are  prolonged 
into  processes  continuous  with  neuroglia  fibres  that  are  soon  lost  in  the 
surrounding  glia-complex.  Where  the  ependyma  lines  the  ventricular  spaces, 
the  cells  are  low  cuboidal  elements  that  constitute  a  continuous  and  single- 
layered  investment,  whose  primary  relation  to  the  surrounding  neuroglia  is 
often  lost  or,  at  best,  obscured. 

The  Nerve  Trunks. — The  component  fibres  of  the  peripheral  nervous 
system  are  assembled  into  larger  or  smaller  cords,  the  "nerves "of  gross 
anatomy,  which  extend  to  various  parts  of  the  body.  Those  that  supply  both 
muscles  and  sensory  surfaces  (integument  or  mucous    membranes),  as,  for 


72 


NORMAL    HISTOLOGY. 


example,  the  median  or  the  mandibular  nerve,  include  three  setS'  of  nerve 
fibres:  (i)  the  efferent  axones  of  the  motor  neurones,  whose  cell-bodies  are 
situated  within  the  spinal  cord  or  the  brain-stem  ;  (2)  the  afferent  dendrites 
of  sensory  neurones  within  the  spinal  and  other  sensory  ganglia;  and  (3)  the 
efferent  axones  of  neurones  within  the  sympathetic  ganglia  that  accompany 
the  spinal  fibres  to  the  periphery  for  the  innervation  of  the  involuntary  mus- 
cle of  the  blood-vessels  and  of  the  skin  and  the  glands. 

The  nerve-fibres,  the  representatives  of  the  three  sets  usually  more  or  less 
intermingled,  are  grouped  into  bundles,  the  funiculi,  which  differ  in  num- 
ber and  diameter  according  to  the  size  of-  the  entire  trunk  that  they  form. 
Each  funiculus  is  surrounded  by  a  definite  sheath  of  dense  connective  tissue, 


Epineunum 


,    Blood-vessels 


%»}t  ~.<iV''-v«k  Perineurium 


fj'  y:'7^  ~~~~  Funiculus  of 

f*  f"  nerve-fibres 


Fig.  97. — Transverse  section  of  small  nerve-trunk  composed  of  loosely  united  funiculi.    X  20. 

the permeiirmm,  which  is  continuous  with  the  delicate  fibro-elastic  tissue  pro- 
longed as  the  endoneurium  between  the  individual  nerve-fibres.  Where  well 
developed,  the  sheath  of  the  funiculus  consists  of  concentric  fibrous  lamellae, 
which  enclose  Xkve perineural  lymph-spaces.  The  latter  are  in  relation  with  the 
lymph-clefts  between  the  nerve-fibres,  on  the  one  hand,  and  with  the  lymphatics 
within  the  interfunicular  tissue  on  the  other.  When,  as  usually  is  the  case,  the 
nerve  is  made  up  of  several  funiculi,  these  are  loosely  bound  together  and  the 
entire  nerve-trunk  so  formed  is  invested  by  a  general  connective  tissue  envelope, 
the  epineurium,  in  which  lie  the  larger  blood-vessels  and  the  lymphatics. 
These  coverings  of  the  nerve-trunk  are  continued  over  its  branches,  even 
over  its  smallest  subdivisions.  The  last  representative  of  these  envelopes  is 
prolonged  over  the  individual  nerve-fibres  as  the  sheath  of  Henle,  which  lies 
outside  the  neurilemma  and  consists  of  flattened  cells  and  delicate  strands  of 
connective  tissue. 

In  cross-sections  of  the  nerve-trunk  (Fig.  98),  the  transversely  cut  indi- 
vidual medullated  nerve-fibres  appear  as  small  circles,  sharply  defined  by  a 
fine  outline  (the  neurilemma),  each  enclosing  a  deeply  stained  dot  (the  axis- 
cylinder  in  section) ;  the  interval  between  the  latter  and  the  neurilemma  cor- 
responds to  the  space  occupied  by  the  myelin  and  usually  appears  clear  and 
unstained,  with  the  exception  of  delicate  and  uncertain  suggestions  of  menir 
branous  septa.     In  contrast  with  the  foregoing  appearance,  is  that  seen  after 


NERVE-TRUNKS. 


73 


the  action  of  osmic  acid  or  special  hematoxylin  staining  (Weigert),  the  med- 
ullary substance  then  exhibiting  a  dark  color  and  appearing  as  a  deeply 
tinted  ring  which  surrounds  the  axis-cylinder.  The  neurilemma-nuclei  are 
occasionally  seen  as  deeply  stained  crescentic  figures  that  partly  encircle  the 


^.>i:ai^2 


mmm-- 


m:^- 


Perineurium 


c 


v.-^- 


"vv^     '~   Endoneuriutn 


Nerve-fibre 


Epineurium 


Blood-vessel 
Fig.  98.— Transverse  section  of  funiculus  composed  of  nerve-fibres  held  together  by  endoneurium  and 
surrounded  by  perineurium.    X  i75' 

nerve-fibres,  lying  beneath  the  neurilemma.  Viewed  in  cross-sections,  the  non- 
medullated  fibres  appear  as  small  irregularly  round  fields  arranged  in  groups 
that  correspond  to  bundles.  When  numerous,  the  latter  are  aggregated  into 
secondary  bundles  between  which  _  . 

,,-■,.  -  .  Epineunum 

extend  delicate  septa  of  connective  / 

tissue,   continuous  with    the   gen-  :  Li^  --3^5^^"^- / 1 

eral  envelope  of  the  nerve-trunk. 
The  fibres  being  nonmedullated, 
their  diameter  is  very  small,  some- 
times less  than  i  //. 

The  Ganglia. — The  cell- 
bodies  of  the  neurones  constituting 
the  sensory  or  afferent  paths  within 
the  peripheral  nerves,  as  well 
as  those  within  the  sympathetic 
(visceral)  nerves,  are  collected  into 
aggregations  known  as  ganglia. 
Familiar  examples  of  the  latter  are 
the  spinal  ganglia  on  the  dorsal 
roots  of  the  spinal  nerves,  certain 
cranial  ganglia  (as  the  semilunar 
[Gasserian]  connected  with  the  fifth  nerve,  the  acoustic  with  the  eighth,  and 
those  on  the  trunks  of  the  seventh,  ninth,  and  tenth  cranial  nerves),  and  the 
sympathetic  ganglia  along  the  gangliated  cords  and  within  the  plexuses  of  the 
sympathetic. 


\      Inter- 
fascicular 
septum 


Fig.  qg. — Transverse  section  of  small  splenic  nerve  con- 
sisting chiefly  of  nonmedullated  fibres.     X  i8o. 


74 


NORMAL   HISTOLOGY. 


A  longitudinal  section  of  a  spinal  ganglion  (Fig.  loo),  which  may  be 
taken  as  a  type  of  such  collections,  shows  the  entire  ovoid  mass  to  be  sur- 
rounded by  a  fibrous  capsule,  continuous  with  the  epineurium  ensheathing 
the  nerves.  Immediately  beneath  the  capsule,  the  ganglion-cells  are  dis- 
posed in  a  fairly  continuous  layer  of  varying  thickness,  while  the  more 
deeply  placed  cells  are  broken  up  into  groups  by  the  tracts  of  nerve-fibres, 
a  small  amount  of  connective  tissue  prolonged  from  the  endoneurium  of  the 
nerve-bundles  and  accompanying  the  blood-vessels  being  also  present.     The 


Spinal  cord^ 


Dorsal  root  (sensory) 


Spinal  ganglion 


Ventral  root  (motor) 


Common  trunk  of  spinal  nerve 
Ventral  or  anterior  primary  division 

Fig.  ioo. — Section  of  spinal  nerve,  showing  its  roots,  ganglion,  common  trunk  and  primary 

divisions.     X  9- 

majority  of  the  ganglion-cells  are  from  60-80  m  in  diameter,  but  some  meas- 
ure as  much  as  170  m,  and  others  as  little  as  25  ix.  In  sections  they  usually 
appear  round  or  oval,  since  only  exceptionally  are  their  processes  to  be  seen, 
as  these  seldom  correspond  with  the  plane  of  section.  Each  nerve-cell  is 
immediately  surrounded  by  a  homogeneous  capsule,  lined  by  flattened  cap- 
sule-cells, which  are  regarded  as  continuations  of  the  neurilemma-cells. 
Outside  the  capsule  a  second  investment,  the  micleated  sheath,  is  usually  to 
be  distinguished.  This  consists  of  connective  tissue  elements  and  is  contin- 
uous with  the  endoneurial  sheath  accompanying  the  nerve-fibres. 

The  sympathetic  ganglia  correspond  in  their  general  structure  with 
those  situated  on  the  spinal  nerves.  They  are  enclosed  by  a  fibrous  capsule, 
from  which  prolongations  of  connective  tissue  pass  into  the  interior  of  the 
ganglion  for  the  support  of  the  nervous  elements.     The  individual  ganglion 


GANGLIA. 


75 


Nerve  fibres,  cut  transversely 


Ner\-e-cell 


cells — unipolar,  bipolar  or  multipolar — are  surrounded  by  nucleated  capsules 
continuous  with  the  neurilemma  of  the  nerve-fibres.  Most  of  the  ganglion  cells 
belong  to  the  sympathetic  efferent  (motor)  neurones,  whose  a.xones  pass  as 
nonmedullated  fibres  to  join  the  nerve- 
trunks  and  finally  end  in  involuntary 
muscle.  Other  neurones,  whose  cell- 
bodies  are  more  or  less  triangular,  are 
distinguished  by  unusually  long  den- 
drites that  pass,  in  company  with  the 
axone,  along  the  connecting  trunk  to  a 
neighboring  ganglion.  Their  termina- 
tion is  uncertain,  but  they  probably  are 
sympathetic  afferent  or  sensory  neu- 
rones. A  third  and  very  infrequent 
variety  of  neurone  possesses  richly 
branched  dendrites  which  form  a  plexi- 
form  arborization  in  the  periphery  of  the 
ganglion,  while  the  axone  enters  a  near- 
by nerve-trunk.  Although  the  axones 
of  the  sympathetic  neurones  for  the 
most  part  are  devoid  of  medullary 
sheath,  and  appear  as  pale  fibres,  this  condition  often  applies  only  to  part  of 
their  course,  since  many  such  processes  temporarily  acquire  a  myelin-sheath 
and  run  for  a  variable  distance  as  medullated  fibres.  The  spinal  efferents, 
which  join  the  sympathetic  by  way  of  the  white  rami  communicantes,  are  also 


Capsule 


Nerve-fibres 


Fig.  ioi. — Section  ot  spinal  ganglion,  showing 
nerve-cells  surrounded  by  nucleated  capsules. 
X  300- 


■  Supporting  tissue 


_  Nonmedullated  nerve- 
fibres 


Ganglion  cells 


^■"^--"*^-*;M 


—1  ' 
Fig.  102. — Portion  of  section  of  sympathetic  (semilunar)  ganglion  from  child.     >(  250. 

medullated  fibres.  Eventually,  they  too  lose  the  myelin-sheath  and,  after  a 
variable  but  short  course  as  nonmedullated  fibres,  end.  in  arborizations  com- 
posed of  naked  axis-cylinders  that  surround  the  sympathetic  ganglion  cells. 
Under  the  paraganglia  are  included  clumps  or  cord-like  collections  of 
cells,  which  are  derived  from  the  formative  areas  of  the  sympathetic  ganglia 
and  share  with  cells  scattered  throughout  the  sympathetic  nerves  and  ganglia 


76 


NORMAL   HISTOLOGY. 


the  peculiarity*  of  being  stained  yellowish  brown  by  solutions  containing 
chromic  acid  or  chromium  salts.  In  recognition  of  this  affinity,  these  ele- 
ments are  known  as  chromaffine  cells  and  regarded  as  related  to  the  sympa- 
thetic system.  Definite  collections  of  such  cells,  associated  with  a  complex 
of  blood-vessels  along  the  course  of  large  arteries,  occur  in  the  carotid  and 
aortic  bodies,  as  well  as  within  the  medulla  of  the  suprarenal  body. 


DEVELOPMENT  OF  THE  NERVOUS  TISSUES. 

Although  the  reader  must  be  referred  to  the  larger  or  special  books  for  a  syste- 
matic and  detailed  description  of  the  development  of  the  nervous  system,  an  under- 
standing of  the  chief  features  of  its  histogenesis  is  so  important  for  an  appreciation  of 


Amniotic  sac 


Closing  neural  canal 


Body-cavity 


Body-cavity 


Visceral  mesoderm    Entoderm  Chorda  Open  gut-tube 


Splanchnopleura 


Fig.   103. — Transverse  section  of  rabbit  embryo  of  about  nine  and  one-quarter  days.    X  80.     Neural 

canal  is  just  closing. 

the  relations  of  its  structural  components,  that  a  sketch  of  these  processes  finds  here 
an  appropriate  place. 

Among  the  very  earliest  phases  of  the  embryo  is  the  formation  of  a  longitudinal 
furrow,  the  neural  groove^  bounded  by  thickened  ectoderm  and  corresponding  with 

the  long  axis  of  the  embryo.  By  the  approxi- 
mation and  fusion  of  its  dorsal  lips,  this  groove 
is  gradually  converted  into  a  closed  tube,  the 
neural  canal.  The  walls  of  this  canal,  from 
which  all  the  essential  nervous  elements  are 
derived,  consist  at  first  of  only  a  few  layers 
of  the  invaginated  ectodermic  epithelial  cells. 
The  latter  actively  proliferate  and  become  con- 
verted into  a  multinucleated  tissue  in  which 
the  cell-boundaries  disappear  and  the  nuclei 
lie  embedded  within  a  general  protoplasmic 
tract  or  syncytiimi.  The  larger  dividing  ele- 
ments, the  germinal  cells,  conspicuous  on 
account  of  the  mitotic  figures,  lie  close  to 
the  lumen  of  the  tube.  Soon  this  continuity  is 
interrupted  by  the  appearance  of  spaces  within 
the  syncytium,  the  cell-substance  being  re- 
solved into  a  delicate  reticulum,  the  myelo- 
spongiwn.  The  meshes  of  the  reticulum 
enlarge,  the  intervening  nucleated  tracts  elon- 
gate, and  the  increasing  nuclei  become  radially  disposed.  Following  these  changes, 
the  elements  next  the  lumen  assume  a  columnar  form  and  radial  arrangement  and 
become  the  primary  ependymal  cells,  while  the  remaining  elements,  the  indifferent 


ilm 

Fig.  104. — Segment  from  lateral  wall  of 
neural  tube  of  pig  embryo  of  5  mm.;  syncyt- 
ium replacing  distinctly  outlined  cells,  a, 
inner  zone;  g,  germinal  cells;  ilm,  internal 
limiting  membrane;  ;«,  peripheral  zone;  r, 
radial  strands  of  cytoplasm.  X  690.  {Har- 
desty.) 


DEVELOPMENT  OF  NERVOUS  TISSUES. 


77 


cells,  increase  by  the  continued  division  of  the  germinal  cells.  The  indifferent  cells 
later  differentiate  into  the  spongioblasts,  from  which  the  characteristic  constituents 
of  the  neuroglia  are  derived,  and  the  neuroblasts,  which  are  directly  converted  into 
the  neurones. 

The  neuroglia  is  evolved  by  the  gradual  transformation  of  the  spongioblasts  and 
their  descendants  into  fibrillae  and  the  production  of  a  more  definite  framework  that 
replaces  the  primary  myelospongium  and  eventually,  in  conjunction  with  the  proc- 


ilm 


'-.-x^— >,<^- 


i?lSfr:*S»^ 


Fig.  105. — Segment  of  wall  of  neural  tube  of  pis:  embryo  of  10  mm.;  radial  strands  (?)  of  syncytium  and 
differentiation  of  ependymal  (a),  nuclear  (5)  and  marginal  {m)  layers;  zVw,  <?/>«,  internal  and  external 
limiting  membrane  ;  g,  dividing  cell ;  p,  pia  mater.    X  690.     {Hardesty.) 

esses  derived  from  the  ependymal  cells,  gives  rise  to  the  completed  supporting  tissue 
(page  70).  The  neurones  are  directly  derived  from  the  neuroblasts.  The  latter  are 
distinguishable  from  the  spongioblasts  as  soon  as  they  are  provided  with  nerve- 
processes.  These  appear  as  outgrowths  from  the  peripherally  directed  and  pointed 
ends  of  the  developing  nerve-cells.  The  first,  and  for  a  considerable  time  the  only, 
processes  which  the  motor  neurones  possess  correspond  to  axones  that  become  the 


Fig.  106. — Transverse  section  of  part  of  developing  spinal  cord  from  pigembr>"Oof  30  mm.;  c,  central  canal ; 
ep,  ependymal  layer;  w,  nuclear  layer;  m,  marginal  layer;  r,  radial  fibres.     X  240.     (Hardesty.) 

axis-cylinders  of  eft"erent  (motor)  nerves.     Subsequently  the  dendrites  grow  out  in 
various  directions  from  the  cell-bodies  of  the  young  neurones. 

The  peripheral  nerves,  according  to  the  teachings  of  His  and  the  views  of  most 
anatomists,  are  essentially  outgrowths  from  the  nerve-cells,  since  the  axis-cylinder  of 
the  entire  nerve-fibre  is  formed  by  the  peripherally  directed  growth  of  the  original 
nerx^e-process  of  the  neuroblast.  The  opposed  opinion,  that  the  nerve-fibre  arises  by 
the  fusion  of  a  number  of  segments,  is  not  in  accord  with  the  most  recent  embryo- 
logical  data.  The  motor  neuroblasts  within  the  spinal  cord  and  the  sensory  cells 
within  the  spinal  ganglia  send  out  processes  of  considerable  thickness,  which  give  rise 
at  their  extremities  to  groups  of  fibrillas.  These  increase  in  thickness  and  length 
and,  in  turn,  at  their  extremities  give  rise  to  new  groups  of  fibrils.  The  latter  proceed 
at  first  as  naked  bundles,  but  soon  become  surrounded  by  the  sheath-cells,  which  are 


78 


NORMAL   HISTOLOGY. 


Fig.  107. — Portion  of  spinal  cord  of  human  embryo,  showing 
development  of  ventral  root-axones  as  outgrowths  from  ventral 
neuroblasts.     X  300.     {After  His.) 


of  mesodermic  origin,  and  enclose  the  3'oung  developing  nerve.  After  a  nerve  has 
become  enlarged  by  the  distal  ingrowth  of  fibrils,  the  sheath-cells  wander  from  the 
peripher}'  among  the  fibrillce,  and  thus  give  rise  to  a  network  that  divides  the  original 

fasciculus  into   a   number  of 
r\  '^'/d^^^^^^^x^  secondarj^  bundles.     The  in- 

terfascicular cells  increase 
rapidly,  the  subdivision  con- 
tinues, and  the  bundles  of 
fibrillce  become  progressively 
smaller  and  more  compact 
until,  surrounded  by  mem- 
branous septa,  they  become 
the  axis-cylinders  of  the  indi- 
\idual  nerve-fibres,  enclosed 
by  the  neurilemma  and  its 
cells.  The  eiidoneurium  ap- 
pears comparatively  late  and, 
hke  the  neurilemma,  is  a  prod- 
uct of  the  mesoderm.  Later 
condensations  of  the  meso- 
dermic tissue  around  the  defi- 
nite bundles  of  nerve-fibres 
and  around  the  entire  nerve- 
trunk  provide  Xh&pe)'ineurium 
and  the  epineurium  respect- 
ively. The  medullary  sheath  is  acquired  comparatively  late,  since  it  does  not  appear 
until  about  the  fourth  month  of  fcetal  life,  some  tracts  within  the  central  nervous  axis, 
indeed,  not  obtaining  the  medullary  coat  until  ■  ■  i... 

after  birth.     It  is  probable  that  formation  of  :;i  ::■•;>'--•' 

the  medullary  substance  is  in  some  way  influ-  '     ^--^-.  •    ■  ■- 

enced  by  the  axis-cj-linder,  resulting  in   the 

deposit  of  the  myelin  droplets  from  the  fluid  •  .  ,  . 

that  surrounds  the  axial   thread.      Thus,  the  .  ^     ^^ 

axis-cylinder  is  derived  from  the  ectoderm, 
the  neurilemma  from  the  mesoderm,  while    sg 
the  medullary  sheath  is  indirectly  from  the 

mesoderm.  ,,  ~        : — ~- ,nz 

The  sensory  ganglia  develop  from  groups 
of  ectodermic  cells  that  form  a  ridge,  the  gan-  '''' 

glion-crest,  on  the  margin  of  either  lipof  the  _^ 

still  open  neural  tube,  just  where  the  general  ~  '''•^ 

ectoderm  passes  into  that  lining  the  groove.      ^  "■      '.■:.'.. 

On  closure  of  the  latter,  the  ganglion-crests 
fuse  into   a  dorsal  wedge-shaped   mass  that        __    ---- — """^^ 
becomes  a  centre  of  proliferation  from  which    ^' 
cells  migrate  outwards  over  the  dorso-lateral        _ 

wall  of  the  tube.  In  consequence,  a  series  of  <'/.;'.:. 

segmentally  arranged  cell-aggregations  ap- 
pears on  each  side  of  the  neural  canal,  these 
collections  being  the  rudiments  of  the  later 
spinal  ganglia.  Within  them  certain  cells  soon 
become  fusiform  and,  assuming  the  role  of 
neuroblasts,  send  out  a  process  from  each  end. 
One  process,  the  axone,  grows  centrally,  while 
the  other,  the  dendrite,  extends  peripherally 

and  becomes  the  chief  part  of  a  sensory  neurone,  the  afferent  nerve-fibre.  The 
sub.sequent  growth  of  the  neurone  is  not  symmetrical,  but  to  one  side,  so  that  the  two 
processes  approximate  and,  finally,  join  the  cell-body  by  a  common  stalk,  the  neurone 
being  thus  converted  into  an  unipolar  ganglion-cell.     The  centrally  directed  processes. 


Fig.  ioS. — Cross-section  of  part  of  dorsal 
region  of  human  embryo,  showing  develof)- 
ment  of  spinal  ganglion  ;  d  z.  v  z,  jti  z,  dorsal, 
ventral  and  marginal  zones  of  early  spinal 
cord  ;  dr,  vr,  dorsal  and  ventral  root-fibres  of 
spinal  nerve  (sn)\  sg-,  spinal  ganglion  on  dor- 
sal root,     X  75. 


NERVE-TERMINATIONS.  79 

later  the  dorsal  root-fibres  of  a  spinal  nerve,  grow  into  the  developing  cord  and  enter 
the  immature  white  matter  to  end,  when  development  is  complete,  at  various  levels  in 
relation  with  the  neurones  within  the  gray  matter  of  the  cord.  The  peripherally  directed 
processes,  on  the  other  hand,  mingle  with  the  a.xones  from  the  motor  neurones  to  form 
the  mixed  nerves  distributed  to  the  various  parts  of  the  body.  The  essential  parts  of 
the  sensory  neurones,  the  cell-body  and  the  processes,  are  derivatives  of  ectodermic 
elements,  while  the  sheaths,  whether  of  the  nerve-cells,  of  the  fibres,  or  of  the  entire 
ganglion,  are  contributions  of  the  mesoderm. 

The  sympathetic  ganglia,  which  include  essentially  three  sets — those  of  the  gan- 
gliated  cords,  those  within  the  prevertebral  plexuses,  and  those  within  the  organs — are 
direct  descendants  of  the  neurogenetic  elements  derived  from  the  developing  spinal 
nerves.  The  earliest  suggestions  of  definite  sympathetic  ganglia  appear  about  the  be- 
ginning of  the  second  fcetal  month  as  aggregations  of  neuroblasts  at  the  distal  ends  of 
the  visceral  rami  of  the  developing  spinal  nerves.  From  these  cells  are  derived  the 
definite  sympathetic  neurones  of  the  gangliated  cords,  as  well  as  those  which  follow 
the  mesial  ingrowth  of  the  spinal  fibres  for  the  production  of  the  prevertebral  and  the 
terminal  ganglia.  The  ganglia  thus  established  constitute  for  a  time  a  series  of  isolated 
nodes.  Subsequently  these  are  connected  by  the  differentiation  of  sympathetic  axones 
which  grow  from  one  young  ganglion  to  the  next  and,  in  conjunction  with  the  spinal 
fibres,  form  the  longitudinal  strands  of  the  gangliated  cords.  Other  sympathetic  neu- 
rones send  axones  centrally  and  so  assist  in  producing  the  efferent  splanchnic  nerves, 
whilst  still  others  send  axones  to  accompany  the  growing  efferent  somatic  spinal  trunks. 

NERVE-TERMINATIONS. 

The  terminations  of  the  fibres  composing  the  peripheral  nerves — the 
axones  of  certain  motor  neurones  situated  within  the  cerebro-spinal  a.xis  and 
within  the  sympathetic  ganglia  and  the  dendrites  of  the  neurones  within  the 
sensory  ganglia — supply  the  apparatus  by  which  the  various  structures  are 
-brought  into  intimate  relation  with  the  central  nervous  system.  Some  of 
these  terminations  convey  impulses  that  produce  various  sensations  (pain, 
pressure,  muscle-sense,  temperature);  others  transfer  impulses  resulting  in 
muscular  contractions.  The  nerve-terminations,  therefore,  may  be  grouped 
according  to  function  into  soisory  and  motor  endings. 

Sensory  Nerve-Endings. 

Since  the  sensory  endings  are  the  more  or  less  modified  terminal  arbor- 
izations of  neurones  whose  cell-bodies  lie  within  the  spinal  and  other  sensory 
ganglia,  such  terminations  are  functionally  the  beginnings  of  the  paths  con- 
ducting sensory  stimuli    to   the  central    nerA^ous 
system.      According  to  their  relations  to  the  sur- 
rounding tissue,  the  sensory  endings  are  broadly 
grouped  m'io  free  and  encapsulated. 

Free  Sensory  Endings. — These  endings 
include  numbers  of  nerve-terminations  found  in 
the  skin  and  the  mucous  membranes,  chiefly 
within  the  epithelium  but  to  some  extent  also 
within  the  connecti\e  tissue,  and  between  the 
fibres  of  voluntary  muscle.  As  a  rule,  the  sensory  J^  ^^dTrmfs^Trabbft  T?"  1"^! 
(afferent)  nerve-fibres  branch  onlv  to  a  limited    ^rai  places  nerve  fibriiise  termi- 

,  '      ..  .      .  .,',,..  nate  in  end-knobs.     (Dogtel.) 

degree  until    near  their   peripheral    destination, 

where  they  undergo  repeated  division,  always  at  a  node  of  Ranvier  and  in 
various  directions.  The  medullary  coat  of  the  main  fibre  is  retained  until  close 
to  its  termination,  although  some  of  its  branches  may  course  as  nonmedullated 
fibres  for  a  considerable  distance  before  ending  or  entering  the  epithelium. 


8o 


NORMAL   HISTOLOGY. 


"<^-  — •^ 


Fig.  1 10. — Tactile  cells  of  Merkel  lyings 
within  interpapillary  epithelium ;  broken 
line  (e)  indicates  junction  of  epithelium 
and  connective  tissue  layer;  (w). nerve 
passing  into  epithelium.  X  i6o.  {Worth- 
mann.) 


In  the  skin — and  the  same  general  plan  applies  to  the  mucous  membranes — 
the  fibres  destined  for  the  epidermis  lose  their  myelin  coat  beneath  the 
epithelium  which  they  enter  as  vertically  coursing  nonmedullated  fibrils. 
Within  the  epithelium  they  break  up  into  delicate   fibrils  which    undergo 

further  division  into  still  finer  varicose  threads 
that  ramify  between  the  deeper  epithelial  cells 
and  terminate  in  free  end-knobs.  Similar, 
but  far  less  numerous  free-endings,  varicose 
and  club-like  in  form,  occur  within  the  con- 
nective tissue  layers  of  the  skin  and  mucous 
membranes.  Conspicuous  ramifications  of 
sensory  fibres  surround  the  hair-follicles, 
lying  upon  the  outer  surface  of  the  glassy 
membrane. 

The  tactile  cells,  found  in  the  deeper 
layers  of  the  epidermis  and  sparingly  within 
the  subjacent  corium,  represent  a  somewhat 
more  differentiated  form  of  intraepithelial 
terminations  and  suggest  transitions  to  the 
more  specialized  end-organs.  In  these  end- 
ings the  nerve-fibrils  terminate  in  cup-shaped 
expansions,  the  menisci,  against  which  rest 
the  tactile  (modified  epithelial)  cells.  The 
latter  may  be  regarded  as  imperfectly  differentiated  neuroepithelium,  highly 
differentiated  examples  of  which  are  seen  in  the  gustatory  cells  in  the  taste  buds, 
in  the  visual  cells  in  the  retina  and  the  auditory  cells  in  the  organ  of  Corti. 
Encapsulated  Sensory  Endings. — In  their  most  highly  developed 
forms,  these  endings  (corpuscula  nervorum  terminalia)  are  represented  by 
large  special  end-organs  in  which  the  terminations  of  the  axis-cylinder  are 
enclosed  within  an  elaborate  laminated  capsule.  The  latter,  however,  is 
more  often  present  as  a  much  simpler  and  thinner  envelope  consisting  of 
strands  of  connective  tissue. 

Transition  forms  between  the  intraepithelial  tactile  cells  above  noted  and 
the  more  specialized  end-organs,  always  within  the  connective  tissue,  are  seen 
in  the  corpuscles  of  Grandry  (not  found  in  man  but  conspicuous  in  the  skin 
covering  the  bill  of  many  water-fowl),  in  which  the  ramifications  of  the  nerve- 
fibrillse  end  within  a  disk-like  mass,  the  tactile 
disk  enclosed  between  large  modified  epithelial 
elements,  the  tactile  cells. 

The  group  of  simpler  encapsulated  end- 
organs  includes  three  well-known  examples  : 
the  tactile  corpuscles,  the  end-bulbs,  and  the 
genital  corpuscles. 

The  Tactile  Corpuscles. — These 
bodies,  also  called  the  corpuscles  of  Meissner, 
are  most  numerous  in  the  corium  of  the  skin 
covering  the  flexor  surface  of  the  fingers  and 
toes.  They  are  found  also  in  the  integument  of  other  regions  possessing 
sensibility  in  a  high  degree,  such  as  the  lips,  margin  of  the  eyelid,  nipple, 
penis  and  clitoris,  as  well  as  on  the  dorsum  of  the  hand  and  foot  and  the 
radial  surface  of  the  forearm.  On  the  volar  surface  of  the  distal  segments 
of  the  fingers,  where  they  are  most  numerous,  some  twenty  are  found  to 
the  square  millimeter.      The  corpuscles  occupy  the  summit  of  the  papillae 


Fig.  III. — Two  corpuscles  of  Grandry 
from  iDill  of  duck ;  nerve  is  seen  enter- 
ing corpuscle  on  right.     X  265. 


SENSORY    NERVE-ENDINGS. 


'-C''\^ 


H 


^i^;. 


and  ridges  of  the  connective  tissue  stratum  of  the  skin  and  He  close  beneath 
the  epidermis,  with  their  long  axises  perpendicular.  They  are  elongated 
irregular  ellipsoids,  often  somewhat  sinuous  in  outline,  and,  in  the  larger 
papillae,  two  may  be  joined  at  the  deeper 
ends  to  form  a  compound  body.  The 
tactile  corpuscles  are  relatively  large, 
being  from  80—150  ,".  long  and  about  one- 
thircl  as  broad.  Depending  upon  its  size, 
each  corpuscle  receives  from  one  to  four 
nerve-tibres,  which  usually  enter  in  the 
neighborhood  of  the  deeper  pole  and,  on 
piercing  the  capsule  and  losing  the  med- 
ullary coat,  di\'ide  into  a  number  of  naked 
axis-cylinders.  These  pass  in  parallel  or 
spiral  windings,  beset  with  varicose  thick- 
enings, between  the  flattened  tactile  cells, 
the  entire  interlacement  being  embedded 
within  a  semifluid  substance  and  enclosed 
by  a  thin  nucleated  fibrous  capsule. 

The  End-Bulbs. — These  endings 
include  a  variety  of  irregularly  spherical 
or  ellipsoidal  bodies  found  in  the  edge  of 
the  eyelid,  the  conjunctiva  and  corneal 
margin,  the  lips  and  oral  mucous  mem- 
brane, the  glans  penis  and  clitoridis  and 
probably  other  parts  of  the  integument 
highly  endowed  with  sensibility.  In  the 
conjunctiva,  they  lie  superficially  placed 
within  the  connective  tissue  near  the  summit  of  the  papillae  and  folds,  where 
such  elevations  exist,  but  always  close  beneath  the  epithelium.  They  vary 
considerably  in  size,  often  being  small  but  sometimes  measuring  from  50- 

100  iJ.  in  diameter.  Usually  a  single  nerve- 
fibre,  exceptionally  two  or  even  more,  enters 
each  bulb,  losing  its  medullary  coat  as  it 
pierces  the  thin  fibrous  capsule.  Within  the 
latter  the  nerve,  now  represented  by  the  naked 
axis-cylinder,  divides  into  from  two  to  four 
branches,  which,  after  describing  several  an- 
nular or  spiral  turns,  give  off  varicose  fibrils 
that  divide  into  the  terminal  threads,  forming 
an  intricate  maze  within  the  semifluid  sub- 
stance (granular  in  preparations)  enclosed  by 
the  fibrous  capsule. 

The  Genital  Corpuscles. — These  end- 
ings are  most  numerous  (from  one  to  four  to 
the  square  millimeter)  in  the  deeper  strata  of 
the  corium  covering  the  glans  penis  and  clito- 
ridis, but  occur  also  in  the  skin  of  the  neigh- 
boring parts  of   the  genitalia.      They  are  of 
irregular  oval  or  lobulated  outline  and  from 
.02— .35   mm.    in  diameter.      They   present    the   same    general   architecture 
as  the  end-bulbs,   but  are  larger  and  possess  a  somewhat  thicker  capsule 
and  contain  a  more   intricate  interlacement  of   the  terminal  nerve-fibrillae. 


Fig.  112.— Corpuscle  of  Meissner  lying  witli- 
in  papilla  of  corium  of  skin  from  finger  ;  only 
deeper  layers  of  overlying  epidermis  are 
shown  ;  n,  entering  nerve-fibre,     a  270. 


Fig.    113. — Two    end-bulbs 
from  human  conjunctiva. 


of    Krause 
{Dogic'l.) 


82 


NORMAL    HISTOLOGY. 


The  latter  are  derived  from  the  subdivision  of  two  or  three  fibres  that,  after 
losing  their  medullary  coat,  enter  near  the  base  of  the  corpuscle.  The 
fibrillae  are  beset  with  varicose  enlargements  and  club-shaped  terminal 
swellings.      The  fibrous  capsule  consists  of  several  connective  tissue  lamellae, 

possessing  flattened  nuclei,  and  encloses  the  semi- 
fluid or  granular  substance  in  which  the  end-arbor- 
izations are  embedded. 

In  contrast  to  the  foregoing  end-organs,  in 
which  the  axis-cylinder  subdivides  into  numerous 
terminal  threads  disposed 
as  more  or  less  elaborate 
intertwinings,  a  second 
group  is  distinguished  by 
the  presence  of  a  laniellated 
capsule  that  encloses  a  cyl- 
indrical core,  the  inner  bulb, 
containing  the  slightly 
branched  axis-cylinder. 
These  endings,  of  which 
the  Pacinian  corpuscle  is 
representative,  are  rela- 
tively large  and  occur  chiefly  in  the  skin  and.  the  serous  membranes. 

A  transitional  form,  connecting  them  with  the  spherical  end-bulbs,  is 
the  cylindrical  end-bulbs.  These  are  found  in  various  parts  of  the 
corium,  the  oral  mucous  membrane,  and  between  the  bundles  of  striped 
muscle  and  of  tendon.  They  are  irregularly  cylindrical,  often  somewhat 
bent,  and  consist  of  a  thin  lamellated  capsule  that  encloses. a  core  of  semi- 
fluid substance  in  which  is  the  centrally  placed  axis-cylinder.  The  latter, 
after  losing  the  medullary  coat  on  entering  the  proximal  pole  of  the  cor- 


FiG.  114. — Genital  corpuscle 
from  integument  erf  penis ;  nerve 
divides  before  piercing  capsule 
and  terminates  in  intricate  end- 
windings.     {Dogiel.) 


Fig.  1T5. — Genital  cor- 
puscle from  integument 
of  human  clitoris.  X  350. 
{IVorthmann.) 


X 

/ 

^- 

'• 

-■\ 

~^. 

\..._. 

Fig.  ii6.— Cylindrical  end-bulbs  attached  to  sensory  nerves  in  parietal  peritoneum  of  man.    {DogieQ 

puscle,  traverses  the  core  with  little  or  no  branching  until  near  the  distal 
pole,  where  it  ends  in  a  single  or  slightiy  subdivided  terminal  enlargement. 
The  Vater-Pacinian  Corpuscles. — These  structures,  also  called  the 
lamellated  corpuscles,  are  large  ellipsoidal  bodies,  from  .5-1.5  mm.  in  length 
and  about  one-third  as  much  in  breadth,  situated  within  the  connective  tissue 
in  many  parts  of  the  body.  In  man  they  are  found  in  the  deeper  layer  of 
the  corium,  especially  in  the  skin  covering  the  palmar  and  plantar  aspects 
of   the  fingers    and  toes  and    the  nipple,  in  the  connective  tissue   in   the 


SENSORY   J^ERVE-ENDINGS. 


83 


vicinity  of  the  joints,  in  tendons,  in  the  muscle-sheaths,  in  the  periosteum, 
in  the  tunica  propria  of  the  serous  membranes  (the  parietal  peritoneum, 
the  mesentery  and  the  pleura),  in  the  neighborhood  of  the  pancreas  and  of 
the  oviduct.      They  are  particularly  large  in  the  mesentery  of  the  cat,  where 


with  the  unaided  eye  as 
2-3  mm.  in  length, 
part  of  the  Pacinian  cor- 
that  contributes  almost 
and  consists  of  from  one 
connective  tissue  lamel- 
the  lamellae  are  sepa- 
plate-like  connective 
pear  as  fusiform  thick- 
lines  marking  the  indi- 


V 


/' 


Fig.  117.- 


-Vater-Pacinian  corpuscles  from  skin  of  finger  i 
A,  longitudinal,  B,  transverse  section  ;  «,  nerve  entering 
capsule  to  reach  inner  bulb.     X  145. 


they  may  be  detected  readily 
oval  pearly  bodies,  sometimes 

The  most  conspicuous 
puscle  is  the  robust  capsule 
the  entire  bulk  of  the  body 
to  three  dozen  thin  concentric 
lae.  The  opposed  surfaces  of 
rated  by  a  single  layer  of  flat 
tissue  cells,  whose  nuclei  ap- 
enings  along  the  concentric 
vidual  lamellae.  The  axis  of 
the  Pacinian  corpuscle  is  occu- 
pied by  a  core  of  semifluid 
substance,  the  inner  bulb,  in 
which  the  naked  axis-cylinder 
is  embedded.  On  joining  the 
proximal  pole  of  the  corpuscle, 
the  fibrous  (Henle's)  sheath 
of  the  nerve-fibre  blends  with 
the  lamellae  of  the  capsule, 
while  the  medullary  coat  is 
retained  during  the  somewhat 
tortuous  path  of  the  fibre 
through  the  capsule  as  far  as 
the  core.  At  this  point  the 
remaining  envelope  of  the  nerve-fibre  disappears,  the  subsequent  part  of  its 
course,  through  the  inner  bulb,  being  as  the  naked  axis-cylinder.  At  a 
variable  distance  from,  but  often  just  before  gaining,  the  distal  pole  of  the 
core,  the  axis-cylinder  divides  into  from  two  to  four  branches,  each  of 
which  terminates  in  a  slightly  expanded  end-knob.  Sometimes  shortly  after 
penetrating  the  capsule,  the  nerve-fibre  splits  into  two  or  more  axis-cylinders, 
which  then  share  in  common  the  envelope  of  semifluid  axial  substance. 

The  Golgi-Mazzoni  corpuscles,  found  in  the  corium  of  the  skin  on 
the  finger-bulbs  and  on  the  external  genital  organs,  in  the  conjunctiva,  and 
in  the  peritoneum,  are  modifications  of  the  ordinary  Pacinian  corpuscles. 
They  differ  from  the  latter  in  being  smaller  and  in  possessing  fewer  lamellae, 
a  relatively  larger  core  and  a  more  branched  axis-cylinder. 

Mention  may  be  made  of  the  corpuscles  of  Herbst,  which,  on  account 
of  their  accessibility,  are  frequent  objects  of  study.  They  are  found  in  the 
velvety  skin  covering  the  bill  and  in  the  tongue  of  water-fowl  and  are  asso- 
ciated with  the  Grandry's  corpuscles  already  mentioned.  They  closely 
resemble  the  Pacinian  bodies  of  mammals,  but  differ  in  being  smaller,  rela- 
tively broader,  and  in  exhibiting  a  double  row  of  oval  nuclei  within  the  inner 
bulb  and  around  the  axis-cylinder. 

Neuromuscular  Endings. — In  addition  to  sensory  nerve-fibres  which 
end  between  the  muscle  fibres  as  free  terminal  fibrils,  voluntary  muscle  is 
provided  with  special  sensory  end-organs,  long  known  as  muscle-spindles, 
probably  concerned  in  transmitting  impulses  that  afford  impressions  as  to 
tension  or  ' '  muscle-sense. ' '     The  neuromuscular  endings  lie  within  the  con- 


84 


NORMAL   HISTOLOGY. 


Sheath 


nective  tissue  surrounding  bundles  of  voluntary  muscle- fibres  and  are  long 
spindle-shaped  structures,  varying  in  length  from  1-5  mm.  or  more  and  in 
width  from  .1-.3  mm.  where  broadest.  They  are 
widely  distributed,  being  present  probably  in  almost 
all  the  skeletal  muscles,  and  are  especially  numerous 
in  the  small  muscles  of  the  hand  and  foot.  They 
have  not  been  found  in  the  muscles  of  the  eye,  some 
of  those  of  the  face,  those  of  the  pharynx,  the  intrinsic 
muscles  of  the  larynx,  some  of  the  perineal  muscles, 
and  the  diaphragm. 

Each  muscle-spindle  consists  of  a  capsule,  com- 
posed of  a  few  concentric  layers  of  fibrous  tissue,  which 
encloses  a  group  of  from  three  to  ten,  but  sometimes 
as  many  as  twenty,  striated 
B  muscle-fibres,      medullated 

nerves,  blood-vessels  and  in- 
terspersed connective  tissue. 
These  intrafjisal  fibres,  as 
the  enclosed  muscle-fibres 
are  called,  differ  from  those 
of  the  adjacent  muscle  in  be- 
ing much  smaller  in  diameter 
and  length,  markedly  tapered 
towards  either  end,  more 
coarsely  but  less  distinctly 
striated  and  in  possessing 
nuclei  within  the  sarcous  sub- 
stance. The  intrafusal  fibres 
collectively  are  surrounded 
by  a  thin  special  fibrous 
envelope,  the  axial  sheath, 
between  which  and  the 
capsule  lies  a  periaxial 
lymph-space.  Each  spindle 
receives  usually  several  med- 


Capsule 


Tendon 

fasciculi 


Capsule 


Fig.   119. — Neuromuscular   ending 
in  transverse  section.     X  370. 


Fig.  118.— /4,  neuromuscular  endins:;  B,  neurotendinous 
ending  in  longitudinal  section,  methylene-blue  staining.  X 
260.     (Drawn  from  preparation  made  by  Professor  Huber.) 

ullated  nerve-fibres,  which,  after  incorporation  of  their  fibrous  sheaths  with 
the  capsule,  pierce  the  latter  at  various  points  and  proceed  to  the  individual 


MOTOR   NERVE-ENDINGS.  85 

muscle-fibres.  After  repeated  division  during  their  course  through  the 
capsule  and  periaxial  space,  the  nerve-fibres  pierce  the  axial  sheath,  lose 
their  medullary  coat  and  terminate  either  as  one  or  more  ribbon-like  branches 
that  encircle  the  muscle-fibres  in  annular  or  spiral  windings,  or,  after  further 
subdi\'ision,  as  branched  telodendria  in  which  the  fibrils  end  in  irregular 
spherical  or  pvriform  swellings. 

Neurotendinous  Endings. — In  their  general  architecture,  these 
end-organs  resemble  the  muscle-spindles.  They  lie  within  the  interfascic- 
ular connective  tissue,  at  the  junction  of  the  muscle  and  tendon,  and  are 
probably  present  in  all  tendons,  although  in  variable  numbers.  Like. the 
neuromuscular  endings,  the  tendon- spindles,  as  they  are  often  called,  are 
long  fusiform  structures,  from  1.-1.5  mm.  in  length,  surrounded  by  a 
fibrous  capsule.  The  latter  encloses  a  group  of  from  eight  to  twenty  intra- 
fusal tendon  fasciculi,  which  are  smaller  and  apparently  less  mature  than 
those  composing  the  surrounding  tendon-tissue.  The  intrafusal  fasciculi 
are  in\-ested  bv  a  fibrous  axial  sheath,  between  which  and  the  capsule  lies 
a  periaxial  lymph-space.  On  reaching  the  spindle,  after  repeated  branching, 
the  meduUated  nerve-fibres  penetrate  the  capsule,  with  which  their  fibrous 
sheaths  blend,  and  undergo  further  division.  The  medullary  coat  is  lost 
after  they  pierce  the  axial  sheath,  the  naked  axis-cylinders  then  breaking  up 
into  smaller  fibrils  that  extend  along  the  intrafusal  fasciculi.  The  terminal 
ramifications,  applied  to  the  surface  of  the  fasciculi,  vary  in  details.  Some 
arise  as  short  lateral  branches  that  partly  encircle  the  fasciculi  and  end  in 
irregular  plate-like  expansions,  while  others  terminate  between  the  smaller 
fasciculi.  The  tendon-spindles  are  probably  concerned  in  appreciating  the 
degree  of  tension  exerted  by  the  pull  of  muscular  contraction. 

The  terminal  cylinders,  or  Ruffini  s  endings,  are  elongated  slightly 
fusiform  end-organs,  which  supplement  the  fine  sensory  nerve-endings  in 
connective  tissue.  They  lie  at  the  junction  of  the  corium  and  the  sub- 
cutaneous layer  of  the  fingers  and  toes.  They  resemble  somewhat  the 
tendon-spindles,  being  provided  with  a  fibrous  sheath  which  surrounds  the 
elaborate  end-arborizations  of  the  entering  nerve-fibre.  The  latter,  some- 
times single  but  often  double,  loses  its  fibrous  sheath  on  penetrating  the 
capsule,  with  which  the  sheath  blends,  and  enters  the  connective  tissue  as  a 
naked  axis-cylinder.  This  subdivides  into  numerous  branches,  which  are 
beset  with  irregular  varicosities  and  end  in  small  club-shaped  expansions. 

Motor  Nerve-Endixgs. 

The  motor  endings  include  (^d)  the  terminations  of  the  axones  of  neurones, 
whose  cell-bodies  (nerve-cells)  are  situated  within  the  motor  nuclei  of  the  spinal 
cord  and  brain-stem,  that  pass  to  voluntary  muscle  ;  (^b)  the  terminations  of 
sympathetic  neurones  that  end  in  involuntary  muscle  and  (r)'  in  cardiac  muscle. 

Endings  in  Voluntary  Muscle.— On  approaching  their  peripheral 
destination,  the  medullated  efferent  nerve-fibres  branch  repeatedly,  each  fibre 
in  this  way  coming  into  relation  with  a  number  of  muscle-fibres.  When  the , 
medullated  nerve-fibre  reaches  the  muscle-fibre  which  it  supplies,  its  medul- 
lary coat  abruptly  ends  and  the  neurilemma  becomes  fused  with  the  sarco- 
lemma,  while  the  axis-cylinder  passes  beneath  the  sarcolemma  to  terminate 
in  an  end-plate.  The  latter  appears  as  an  oval  field,  from  40-60//  in  its 
longest  diameter,  which  is  applied  to  the  surface  of  the  muscle-substance. 
In  profile  it  often  shows  as  a  slight  elevation.  Embedded  within  a  nucleated 
sheet  of  granular  protoplasm,  the  sole  plate,  lie  the  terminal  arborizations  of  the 


\ 

86\ 


NORMAL    HISTOLOGY. 


axis-cylinder,  formed  of  irregular  varicosities  and  club-shaped  ends.      Usu- 
ally each  muscle-fibre  is  provided  with  a  single  motor  end-plate,  which  may 

he   at  an  equal   or  unequal   dis- 
Nerve.___l  tance  from  the  ends  of  the  fibre. 

Exceptionally  two  end-plates  are 
found    on    one    muscle-fibre,    in 
W'  which  case   the  end-organs  usu- 

ally lie  near  each  other. 

Endings   in    Involuntary 
Muscle. — The    terminations    of 


.End 
plates 


Fig.  120. — Motor  nerve-endings  in  striated  muscle;  bundle 
of  nerve-fibres  separates  to  supply  the  individual  muscle-fibres. 
X  135- 


Fig.  121. — Motor  nerve-ending 
in  striated  muscle ;  terminal  ar- 
borization of  axis-cylinder  lies 
beneath  sarcolemma  embedded 
in  granular  sole-plate.     X  500. 


the  sympathetic  axones  supplying  the  nonstriated  muscle  are  comparatively 
simple.      The  cell-bodies  of  the  neurones  contributing  the  immediate  fibres 
of  distribution  to  visceral  muscle  usually  occupy  the 
nodal  points  (microscopic  ganglia)  of  plexuses  within 
the  walls  of  the  organs,  from  which  bundles  of  non- 
medullated  nerve-fibres    extend  to  and  surround  the 
muscle-bundles.      Entering  the  latter,  the  nerve-fibres 
divide  into  delicate  varicose  threads  that  pass  between 
the  muscle-cells,  parallel  with  their  long  axes.      As  they 
course  within  the  intercellular  sub- 
stance, the  varicose  fibrils  give  oH 
short  lateral  branches  that  end,  as 
does  also  the  parent  fibril,  on  the 
surface  of  the  muscle-cells  in  minute 
terminal  knobs. 

Endings  in  Cardiac  Mus- 
cle.— These,  also  the  terminations 
of  sympathetic  neurones,  include 
varicose  nerve-fibrils  which  may  be 
followed  between  the  muscle-fibres. 
During  this  course  side  branches  arise 
which,  as  well  as  the  main  fibril,  ter- 
minate on  the  muscle  elements  in 
endings  of  varying  complexity.  In 
some  cases  these  are  merely  minute 
simple  end-knobs,  resembling  those  in  involuntary  muscle;  in  other  cases 
they  are  more  elaborate  and  of  a  group  of  secondary  fibrillae  bearing  nodular 
endings,  the  whole  recalling  somewhat  the  motor  end-plates  in  striated  muscle. 


Fig.  122. — Motor 
nerve-ending  in  in- 
voluntary muscle. 
{Huber.) 


Fig.   123. — Motor  nerve- 
ending  in  cardiac   muscle. 

{Smirnow.) 


THE  VASCULAR  SYSTEM. 

The  vascular  or  circulatory  system  includes  the  organs  immediately 
concerned  in  conveying  throughout  the  body  the  fluids  which  bring  to  the 
tissues  the  nutritive  substances  and  oxygen  necessary  for  their  metabolism 
and  carry  away  from  them  to  the  excretory  organs  the  waste  products  formed 
during  metabolism. 

The  system  is  composed  of  two  parts,  the  one  consisting  of  organs  in 
which  circulates  the  blood,  while  the  organs  of  the  other  contain  a  colorless 
or  white  fluid  known  as  lymph  or  chyle.  The  former  of  these  subsystems 
is  the  blood-vascular  system  and  the  latter  is  the  lymphatic  system.  ■  Since, 
however,  the  two  systems  communicate  and  the  lymphatic  system  develops 
as  an  outgrowth  from  the  blood-vascular,  it  is  evident  that  they  are 
intimately  associated  both  anatomically  and  embryologically. 


THE  BLOOD-VASCULAR  SYSTEM. 

The  blood-vascular  system  consists  of  (i)  a  system  of  canals  known 
as  the  blood-vessels,  traversing  practically  all  parts  of  the  body,  and 
(2)  of  a  contractile  organ,  the  heart,  by  whose  contractions  the  blood 
is  forced  through  the  vessels.  The  latter,  in  turn,  are  divisible  into:  (^) 
the  arteries,  which  carry  the  blood  from  the  heart  to  the  tissues;  ((5)  the 
capillaries,  exceedingly  fine  vessels  which  form  a  network  in  the  tissues;  and 
(c)  the  veins,  which  return  the  blood  from  the  tissues  to  the  heart. 

General  Structure  of  Blood-Vessels, — Although  passing  into  one 
another  insensibly  and  without  sharp  demarcation,  typical  arteries,  capillaries 
and  \'eins  present  such  characteristic  histological 
pictures  that  they  are  readily  distinguished.  All 
blood-vessels,  including  the  heart,  possess  an  endo- 
thelial  lining  which  may  constitute  a  distinct  inner 


Fig.  124.—^,  endothelial  lining  of  small  arterj'  after  silver-staining.     X  200.     B,  endothelial  cells  more 

highly  magnified.     X  300. 

coat,  the  tiinica  intivia,  or,  as  in  the  capillaries,  even  the  entire  wall  of  the 
vessel.  Usually,  however,  the  intima  consists  of  the  endothelium  reinforced 
by  a  variable  amount  of  fibro-elastic  tissue,  in  which  the  elastica  predominates. 
Except  within  the  walls  of  capillaries,  external  to  the  intima  lies  a  thick  middle 
coat,  the  tunica  media,  composed  of  intermingled  lamellae  of  in\'oluntary  mus- 
cle and  elastic  tissue  and  fine  fibrous  fibrillae.  Outside  the  media  follows  the 
ticnica  externa  or  adventitia,  which,  although  usually  thinner  than  the  middle 

(87) 


NORMAL    HISTOLOGY. 


coat,  is  of  exceptional  strength  and  toughness.  It  should  be  noted  that  the 
endothelial  tube  is  the  fundamental  and  primary  structure  in  all  cases,  the 
other  coats  being  secondary  and  variable  according  to  the  size  and  character 
of  the  vessel.  The  customary  division  of  the  wall  into  the  three  coats  is 
more  or  less  artificial  and  in  the  larger  vessels  often  uncertain.  The  recog- 
nition of  an  inner  endothelial  and  an  outer  musculo-elastic  coat  frequently 
more  closely  corresponds  to  the  actual  arrangement  than  the  conventional 
subdivision  into  the  three  tunics. 

The  endothelial  lining  of  the  arteries  consists  of  elongated  spindle- 
shaped  plates  united  by  sinuous  Unes  of  cement  substance,  which,  after 
silver-staining,  map  out  the  contours  of  the  cells  with  diagrammatic  clearness 
(Fig.   124).   Within  the  veins,  the  endothelial  plates  are  shorter  and  broader 

Endothelium 


Intima 


Media       -  ^  -   ' 


■^•^^if  3^'^».s•'*.' 


Involuntary  muscle 


—  Elastic  tissue 


i:^/- 

^!^. 


Z External  elastic 

membrane 


Adventitia 


-  Elastica 
Vasa  vasorum 


J.  ;G.  125. — Transverse  section  of  artery  of  medium  size.     X  150- 

than  in  the  arteries;  The  demarcation  of  the  endothelium  into  distinct  cells 
is  less  evident  in  the  capillaries  than  in  the  larger  vessels,  in  some  cases  a 
continuous  syncytial  sheet  replacing  definitely  outHned  plates.  The  presence 
of  small  oval  nuclei  is  readily  demonstrated  by  suitable  stains. 

The  involuntary  muscle  varies  in  amoimt  from  the  imperfect  single 
layer  of  muscle-cells  found  in  the  arterioles  to  the  robust  muscular  coat  of  many 
lamellae  in  the  larger  arteries.  It  is  relatively  best  developed  in  arteries  of 
medium  size,  where  the  muscle  occurs  in  distinct  broad  or  sheet-like  bundles 
between  the  strands  of  elastic  tissue.  The  component  fibre-cells  are  short 
and  often  branched  and,  for  the  most  part,  circularly  disposed.  _  The  distri- 
bution of  the  muscular  tissue  is  much  less  regular  and  constant  in  the  veins 
than  in  the  arteries,  since  in  many  it  is  scanty,  in  some  entirely  wanting,  and 
in  a  few  excessive,  occurring  as  both  circular  and  longitudinal  layers.      The 


THE   ARTERIES.  89 

striated  muscle  found  in  the  large   \-essels   communicating  with   the   heart 
resembles  that  of  the  cardiac  wall,  with  which  it  is  continuous. 

Connective  tissue  is  represented  in  the  arteries  and  veins  by  both 
fibrous  and  elastic  tissue.  The  former  is  present  usually  as  bundles  of  white 
fibres  between  the  other  components  of  the  vessel- wall.  The  elastic 
tissue  is  conspicuous  in  all  arteries,  save  the  smallest,  and  in  many  veins. 
It  presents  all  variations  in  amount  from  loose  networks  of  delicate  fibres 
in  the  smaller  vessels  to  robust  plates  and  membranes  in  the  largest  arteries. 
Within  the  intima  of  the  latter,  the  elastic  tissue  often  occurs  as  sheets 
broken  by  pits  and  perforations,  which  are,  therefore,  known  as  fenestrated 
membranes. 

Nutrient  blood-vessels  are  present  in  the  walls  of  all  the  larger  vessels, 
down  to  those  of  i  mm.  in  diameter,  and  provide  nourishment  for  the  tissues 
composing  the  tubes.  These  vasa  vasorum,  as  they  are  called,  are  usually 
branches  from  some  neighboring  artery;  their  favorite  situation  is  the  external 
coat,  within  which  they  break  up  into  capillaries  that,  in  the  larger  vessels, 
invade  the  media,  but  never  the  intima.  The  blood  from  the  vessel-wall  is 
collected  by  small  veins  that  accompany  the  nutrient  arteries,  or,  as  in  the  case 
of  the  veins,  empty  directly  into  the  venous  trunk  in  whose  walls  they  course. 

The  lymphatics  are  represented  by  networks  of  surrounding  canals 
within  the  adventitia.  In  certain  situations,  conspicuously  in  the  brain  and 
the  retina,  the  blood-vessels  are  enclosed  by  lymph-channels,  \\\^ perivascular 
lymph-sheaths,  that  occupy  the  outer  coat. 

The  nerves  distributed  to  the  walls  of  blood-vessels,  except  those  of  the 
nervous  substance  of  the  brain  and  spinal  cord  in  which  nerves  are  wanting, 
are  numerous  and  include  both  efferent  and  afferent  fibres.  These  form  a 
perivascular  plexus  around  the  vessel  from  which  motor  (sympathetic)  fibres 
pass  to  the  involuntary  muscle,  while  the  sensory  fibres  end  within  the  outer 
and  inner  tunics.  Special  nerve-endings  have  been  described  in  both  the  ex- 
ternal and  internal  tunics. 

The  Arteries. — In  cross-sections  of  arteries  of  medium  size,  after  the 
usual  methods  of  preservation  which  cause  some  contraction  of  the  vessels,  the 
intima  presents  a  plicated  contour,  since  it  follows  the  foldings  of  the  internal 
elastic  membrane.  The  latter  appears  as  a  conspicuous  corrugated  light  band, 
marking  the  outer  boundary  of  the  inner  tunic.  The  lining  endothelial  cells 
are  so  thin,  that  in  profile  their  presence  is  indicated  chiefly  by  the  slightly 
projecting  nuclei.  The  endothelium  and  the  elastic  membrane  are  separated 
by  a  thin  layer  of  fibro-elastic  tissue.  The  media,  thick  and  conspicuous, 
consists  of  circularly  disposed  flat  bundles  of  involuntary  muscle  separated 
by  plates  of  elastic  tissue.  After  the  usual  stainings,  these  plates  appear 
light  and  almost  uncolored,  but  after  selective  dyes,  as  orcein,  the  elastica  is 
very  conspicuous  (  Fig.  1 26) .  Delicate  bundles  of  fibrous  tissue  lie  among  the 
musculo-elastic  strands.  The  outer  boundary  of  the  media  is  marked  by  a 
more  or  less  distinct  external  elastic  memtrane.  The  adventitia  varies  in 
thickness,  being  relatively  better  developed  in  the  medium  sized  arteries  than 
in  the  larger  ones.  It  consists  of  bundles  of  fibrous  tissue  intermingled  with 
elastic  fibres  of  varying  thickness.  Sometimes  scattered  bundles  of  longi- 
tudinal muscle  are  present.  The  adventitia  contains  the  nutrient  blood-ves- 
sels and  the  chief  lymph-channels  and  blends  with  the  surrounding  areolar 
tissue  without  sharp  demarcation. 

Followed  towards  the  capillaries,  the  coats  of  the  artery  gradually 
diminish  in  thickness.  The  elastic  tissue  becomes  progressively  reduced 
until  it  entirely  disappears   from  the  middle  coat,   which  then  is  a  purely 


90 


NORMAL   HISTOLOGY. 


muscular  tunic.  Before  the  capillary  is  reached,  the  muscle  is  reduced  to  a 
single  layer  of  cells,  which  in  turn  gives  place  to  groups  of  muscle-cells 
that  partially  wrap  around  the  vessel.  After  the  disappearance  of  the  muscle- 
cells  the  blood-vessel  has  become  a  true  capillary.      The  adventitia  shares  in 


External  elastic 

membrane 


Adventitia 


Fig.  126.— Transverse  section  of  artery  of  medium  size,  stained  to  show  elastic  tissue.    X  100, 

the  general  reduction,  and  in  the  smallest  arteries  consists  of  only  a  few  fibro- 
elastic  strands  outside  the  scattered  groups  of  muscle-cells. 

In  the  large  arteries  chiefly  the  intima  and  media  thicken.     Although 
the  inner  coat  greatly  increases  and  contains  a  large  amount  of  fibrous  tissue 


Fig.  127. — Small  arteries  in  which  muscular  coat  is  reduced  to  single  layer  of  cells.  X  isl- 
and elastica,  a  conspicuous  internal  elastic  membrane,  as  seen  in  the  smallest 
vessels,  is  lacking.  The  character  of  the  thickened  media  also  changes,  the 
muscle  being  relatively  reduced  and  overshadowed  by  the  excessive  amount 
of  fibro-elastic  tissue,  which  confers  a  more  compact  and  denser  character  to 
the  wall  of  the  vessel.     The  adventitia,  while  proportionately  thinner  than 


THE  VEINS. 


91 


in  arteries  of  medium  size,  is  also  increased,  and  consists  of  robust  fibres  and 
plates  of  elastica,  many  of  which  are  longitudinal,  and  strong  bundles  of  fibrous 
tissue.  Exceptionally,  among  the  lower  animals  if  not  in  man,  scattered 
bundles  of  involuntary  muscle  are  found  within  the  external  tunic.  In  the 
roots  of  the  aorta  and  pulmonary  artery,  the  media  consists  chiefly  of  striated 


^  liitLma 


'-^  Media 


Adventitia 


Fig.  128. — Transverse  section  of  abdominal  aorta.     X  90. 

muscle  resembling  the  myocardium  with  which  it  is  continuous,  both  vessels 
having  been  derived  from  the  anterior  segment  of  the  primary  heart-tube. 
The  Veins. — Although  the  walls  of  the  veins  are  thinner  than  those 
of  the  corresponding  arteries,  their  thickness  for  veins  of  a  given  diameter  is 
not  constant,  owing  to  the  frequent  irregularity  in  the  composition  of  the 
tunics.  In  consequence  of  the  smaller  amount  of  muscular  and  elastic  tissue 
that  they  contain,  veins  are  generally  more  flaccid  and  less  contractile  than 
the  arteries  which  they  accompany.  In  veins  of  medium  size,  the  intima 
consists  of  the  lining  endothelium,  the  cells  of  which  are  broad  and  short,  a 


92  NORMAL    HISTOLOGY. 

thin  layer  of  fibrous  tissu©  and  networks  of  elastic  fibres.  A  distinct  internal 
elastic  membrane  is  seldom  present.  In  some  veins,  as  the  cephalic,  basilic, 
mesenteric,  iliac,  femoral  and  saphenous,  the  intima  contains  bundles  of 
involuntary  muscle.  The  vtedia,  the  most  variable  coat  of  the  vein-wall, 
consists  of  circularly  disposed  thin  sheets  of  muscular  and  fibro-elastic  tissue, 
reinforced  by  longitudinal  strands  of  fibro-elastic  tissue  and,  sometimes, 
muscle.  In  certain  veins,  as  the  saphenous,  deep  femoral  and  popliteal, 
these  longitudinal  strands  constitute  a  distinct  zone  beneath  the  intima. 
While  in  the  larger  veins  the  intima  is  only  exceptionally  increased,  as  in 
the  hepatic  portion  of  the  inferior  vena  cava  and  the  portal  vein,  the  media 
is  often  markedly  thickened.  This  increase  is  due  chiefly  to  excess  of  the 
elastic  and    fibrous  tissue,   the   muscle   remaining    proportionately   scanty. 

Intima   - ^-^0 


Media 


':^ 


Fig.  129. — Transverse  section  of  vein  of  medium  size.     X  250. 

The  Splenic  and  portal  veins,  however,  are  particularly  rich  in  muscular 
tissue.  On  the  other  hand,  the  media  may  be  almost  wanting,  as  in  the 
greater  part  of  the  inferior  vena  cava  and  the  larger  hepatic  veins,  or  entirely 
disappear,  as  in  the  superior  vena  cava  and  in  the  veins  of  the  pia  and  dura 
mater,  of  the  retina  and  of  bone.  The  valves,  with  which  many  veins  are 
provided,  consist  of  paired  crescentic  projections  of  the  intima,  covered  on 
both  sides  with  endothelium.  The  two  layers  of  endothelial  plates  are 
separated  by  a  thin  stratum  of  delicate  fibrous  tissue,  which  contains  a  dense 
network  of  elastic  fibres  beneath  the  inner  endothelium. 

The  Capillaries. — The  most  favorable  arrangement  for  efficient  nutri- 
tion is,  manifestly,  one  insuring  the  passage  of  the  blood-stream  in  intimate 
relations  with  the  tissue-elements  and  at  a  reduced  rate  of  speed.  These 
requirements  are  met  in  the  capillaries,  whose  collectively  increased  calibre 
and  thin  walls  favor  slowing  of  the  blood-current  and  the  passage  of  the 
plasma  and  oxygen  into  the  surrounding  tissues.  The  walls  of  the  capil- 
laries consist  of  only  the  lining  plates,  the  entire  vessel  being  in  fact  a  delicate 
endothelial  tube.  The  cells  composing  the  latter  are  elongated  lanceolate 
plates,  possessing  oval  nuclei,  united  by  narrow  lines  of  cement  substance. 
Although  the  transition    from    the  arterioles  is  usually  gradual,   the    final 


THE   CAPILLARIES.  93 

disappearance  of  the  arterial  muscle-cells  marks  the  beginning  of  the  true 
capillaries.  The  passage  of  the  latter  into  the  veins  is  less  definite,  since 
muscular  tissue  is  wanting  in  both  the  capillaries  and  the  smaller  veins.  In 
the  smallest  capillaries,  two  endothelial  plates  may  suffice  to  encircle  the 
entire  lumen  ;  in  the  larger  three  or  four  cells  may  be  required  to  complete 
the  vessel.  Although  preformed  openings  (stomata)  in  the  walls  of  the 
capillaries  do  not  exist,  the  escape  of  leucocytes  and,  under  certain  condi- 
tions, also  of  red  blood-cells  (diapedesis)  and  of  small  particles  of  foreign 
substances,  takes  place  between  the  endothelial  plates.  In  some  capillaries, 
as  in  those  of  the  choroid,  liver  or  renal  glomeruli,  the  usual  demarcation  of 
the  wall  into  distinct  cells  is  wandng,  the  individual  endothelial  plates  being 
replaced  bv  a  continuous  nucleated  sheet  or  syncytium.      When  capillaries 

1-  >  xm 


t,  I., 


Capillary  ^^%'    ^ '    V^-^^r::,?!^"**?!.*  V  ,  ^/       ' 


/ 


Vein 


Fig.  130. — Capillaries  arising  from  arteriole  and  ending  in  small  vein  ;  from  the  omentum.     ^'  150. 

course  in  dense  fibrous  tissues,  not  uncommonly  the  vessel  is  accompanied 
by  ensheathing  delicate  strands  of  connective  tissue,  the  adventitia  capillay-is. 

In  certain  organs,  conspicuously  in  the  liver,  the  ultimate  blood-vessels 
arise  by  the  invasion  and  subdivision  of  the  large  primary  blood-channel  by 
the  developing  tissue-cords.  Such  blood-vessels  are  known  as  sinusoids 
(Minot)  and  differ  from  ordinary  capillaries  in  connecting  entering  (afferent) 
and  emerging  (efferent)  vessels  of  the  same  nature,  both  being  either  venous 
or  arterial.  Capillaries,  on  the  contrary,  establish  communication  between 
arteries  and  veins.  In  consequence  of  the  invagination  and  intergrowth 
which  takes  place  between  the  original  blood-channel  and  the  tissue  of  the 
developing  organ,  the  endothelium  of  the  sinusoids  has  an  unusually  intimate 
relation  to  the  cords  of  tissue-cells,  little  or  no  connective  tissue  inter^'ening. 

The  capillaries  are  arranged  usually  as  networks,  the  component  chan- 
nels of  which  are  of  fairly  constant  diameter  within  a  particular  tissue. 
During  life,  it  is  probable  that  none  are  too  small  to  permit  the  passage  of 
the  red  blood-cells,  while  many  admit  two  or  even  three  such  elements 
abreast.  Their  usual  diameter  varies  between  8  and  20  i)..  The  capillary 
networks  in  various  parts  of  the  body  differ  in  the  form  and  closeness  of  their 
meshes,  since  these  details  are  influenced  by  the  arrangement  of  the  elements 
and  by  the  function  of  the  structures  supplied.  Thus,  in  muscles,  tendons 
and  nerves  the  meshes  are  elongated  and  narrow;  in  glands,  the  lungs  and 


94  NORMAL    HISTOLOGY. 

adipose  tissue  they  are  irregularly  polygonal ;  in  the  liver-lobules  they  con- 
verge; while  in  the  subepithelial  papillae  of  the  skin  and  mucous  membranes 
the  capillaries  commonly  form  loops.  In  general,  the  greater  the  functional 
activity  of  an  organ,  the  closer  is  its  capillary  network.  Organs  actively 
engaged  in  excretion,  as  the  kidneys,  or  in  the  elimination  of  substances 
from  the  blood,  as  the  lungs  and  liver,  as  well  as  organs  producing  substances 
direcdy  entering  the  circulation  (organs  of  internal  secretion),  as  the  thyroid 
gland,  are  provided  with  exceptionally  rich  and  close  networks. 

THE  BLOOD. 

The  fluid  circulating  within  all  parts  of  the  blood-vascular  system 
consists  of  a  clear,  almost  colorless  plasma  or  liquor  sajiguinis,  in  which  are 
suspended  vast  numbers  of  small  free  corpuscular  elements,  the  blood-cells. 
The  latter  are  of  two  chief  kinds,  the  colored  cells  or  erythrocytes,  and  the 
colorless  cells  or  leucocytes.  The  characteristic  appearance  of  the  blood  is 
due  to  the  presence  of  hemoglobin  contained  within  the  erythrocytes  which, 
while  individually  only  faintly  tinted,  collectively  impart  the  familiar  hue,  as 
well  as  a  certain  degree  of  opacity.  That  the  characteristic  pigment  is 
limited  to  the  cells  is  shown  by  the  lack   of  color  and  transparency  of  the 

plasma    when    examined    under    the 

.-ji,— ^v^   '_^  microscope,   although  to  the  unaided 

_^  \       \  I        •  eye  the  blood  appears  uniformly  red 

f     '  "-■ y   \        //"^  z'  and  somewhat  opaque. 

/-^  \  The    Colored   Blood-Cells. — 

/  '  As  usually  seen,  the    mature  colored 

^-^     V^_^  blood-cells,    erythrocytes   or   red    cor- 

^-^  ptiscles,   of    man  and  other  mammals 

'  ■"^-x  (except   those    of    the   camel   family, 

, — ^  ....„      which    are   elliptical    in    outline)    are 

,j         >•  i'^      small  biconcave  circular  nonnucleated 

V,_>'/''^     \    (  '"        disks,  with  smooth  contour  and  rounded 

\^J_,    \,  edges.      When  viewed  by  transmitted 

f"      ,  light,  the  individual  ' '  red ' '  cells  pos- 

\,^_^'^  (       ]        ,        '  sess    a  pale  greenish-yellow  tint,  and 

^*— ^/^"^'  only  when  they  are  collected  in  masses 

or  in  several   layers  is  the  distinctive 

Fig.  131. — Human  colored  blood-cells,  spread  into      11 ^1   „^i^„      ^,r\A^.^t-  T"U^      ^^^,,1;,^.- 

a  single  layer  and  dried.   X  iooo.  blood-color     evident.       1  he     peculiar 

form  of  the  corpuscle  as  ordinarily 
seen — biconcave  in  the  centre  and  biconvex  at  the  margin — renders  accurate 
focussing  of  all  parts  of  its  surface  at  one  time  impossible,  the  cell 
appearing,  according  to  focal  adjustment,  either  as  a  dark  ring  enclosing  a 
light  centre  or  vice  versa.  Viewed  in  profile,  the  disk  presents  a  figure  some- 
what resembling  a  dumb-bell,  the  thicker  margins  of  the  cell  being  connected 
by  the  thinner  concave  centre.  Although  the  biconcave  discoidal  form  of 
the  mammalian  erythrocytes  is  the  one  ordinarily  exhibited,  there  is  evidence 
for  believing  that  within  the  circulation,  during  life  and  in  carefully  fixed 
preparations,  the  red  blood-cells  are  cup-shaped,  similar  to  spheres  more 
or  less  deeply  indented. 

The  structure  of  the  red  blood-cell  has  long  been  and  still  is  a  subject 
of  dispute.  According  to  one  view,  the  cell  consists  of  a  soft  tenacious 
envelope  enclosing  a  fluid  contents  containing  the  coloring  matter,  the  hemo- 
globin.     The  other  view  regards  the  corpuscle  as  composed  of  an  extremely 


THE   ERYTHROCYTES. 


95 


delicate  spongy  stroma,  containing  the  hemoglobin,  but  without  a  distinct 
investing  membrane.  It  seems  probable  that  although  no  definite  envelope 
is  present,  in  the  sense  of  a  distinct  cell- wall,  a  peripheral  condensation  of 
the  semifluid  and  hemoglobin-containing  stroma  exists. 

The  average  diameter  of  the  discoidal  red  blood-cells  of  man  is  7. 8  /x, 
some  corpuscles  measuring  as  little  as  4.5  ,a  and  others  as  much  as  9.5  ,«. 
Their  thickness  is  about  1.8  //.  The  average  diameter  of  the  cup-shaped 
corpuscles  is  7  ,a  and  their  thickness  4  ij.  (Lewis).  It  is  probable  that  the 
average  size  is  uninfluenced  by  sex  and  is  constant  for  all  races.  The 
number  of  red  cells  contained  in  one  cubic  millimeter  of  normal  human  blood 
is  approximately  5,000,000  in  the  male  and  something  less  (4,500,000)  in 
the  female.  The  number  of  corpuscles  is  practically  the  same  whether  the 
blood  be  taken  from  the  arteries,  capillaries  or  veins,  but  is  lower  in  the 
blood  from  the  lower  extremity  than 
from  the  upper,  owing  to  the  larger 
proportion  of  plasma  in  the  more 
dependent  parts  of  the  body.  In 
general,  the  red  blood-cells  of 
mammals  are  small  and  their  size, 
which  greatly  varies  in  different 
orders,  bears  no  relation  to  that  of 
the  animal.  The  corpuscles  of  man 
are  among  the  largest  and  exceeded 
by  only  those  of  the  elephant  (9.4 
[J.)  and  of  the  two-toed  sloth.  The 
human  cells  are  approximated  in  size 
by  those  of  some  small  mammals — 
guinea-pig  (7.5  ix),  dog  (7.3  ij.), 
rabbit  (6.9  fx)  and  cat  (6.5  ju). 
Those  of  many  familiar  animals,  as 
the  horse,  hog,  sheep  and  goat,  are  distinctly  smaller.  The  smallest  cor- 
puscles (2.5  (i)  are  those  of  the  musk-ox.  The  positive  recognition  of 
human  blood,  as  differentiated  from  that  of  some  of  the  domestic  animals,  by 
measurement  of  the  red  cells,  is  uncertain  and  often  impossible.  The  non- 
nucleated  mature  red  cells  are  the  distinguishing  characteristic  of  mammalian 
blood,  the  red  cells  of  the  other  vertebrates  being  nucleated  and,  with  few 
exceptions,  large  oval  elements.  The  largest  red  cells  are  found  in  the  tailed 
amphibians;  those  of  the  amphiuma  are  the  largest  known  and  attain  the 
gigantic  length  of  80  p.. 

After  fresh  blood  has  been  distributed  as  a  thin  layer  and  allowed  to 
remain  unshaken  for  some  minutes,  the  red  cells  exhibit  a  tendency  to 
become  arranged  in  columns,  with  their  broad  surfaces  in  contact,  similar  to 
piles  or  rouleaux  of  coin.  If  the  stratum  of  the  blood  be  thin,  the  red  cells 
usually  later  separate,  but  they  sometimes  retain  their  columnar  grouping. 
The  erythrocytes  are  very  sensitive  to  reagents  and  conditions  and  readily 
undergo  change  and  distortion.  Exposure  to  even  a  current  of  air  often 
produces  conspicuous  effects.  Alterations  in  form  result  from  the  action  of 
solutions  of  lower  or  higher  density  than  that  of  the  normal  plasma.  The 
latter  is  conveniently  substituted  by  a  "  normal"  (.85%)  solution  of  sodium 
chloride.  If  the  proportion  of  salt  be  reduced,  the  corpuscles  swell,  at  first 
losing  their  concavity,  then  assuming  the  spherical  form,  parting  with  their 
hemoglobin  and  becoming  colorless.  When  subjected  to  saline  solutions 
denser  than  the  "normal,"  the  exterior  of  the  corpuscles  becomes  irregular 


Fig.  132. — Human  blood  corpuscles  ;  two  leucocytes 
are  seen  among  the  red  cells,  most  of  which  are 
grouped  in  rouleaux.      ■   625. 


96 


NORMAL  HISTOLOGY. 


and  beset  with  knob-like  projections,  or  crenated ;  increased  concentration  of 
the  solution  leads  to  marked  shrinkage  and  distortion,  until  the  cells  lose  all 
resemblance  to  their  usual  form.  Certain  reagents,  as  water,  aqueous  dilutions 
of  acetic  acid  and  ether,  promptly  decolorize  the  erythrocytes  by  extraction 
of  the  hemoglobin.      Alkaline  solutions  completely  destroy  the  red  cells. 

The  Colorless  Blood-Cells. — The  colorless  cells  observed  within  the 
blood  are  probably  only  incidentally,  not  genetically,  related  to  the  erythro- 
cytes; further,  they,  in  part  at  least,  primarily  circulate  within  the  lymph- 
vascular  system,  from  which  they  are  poured  into  the  blood.  They  are  not 
confined,  however,  to  the  blood-  and  lymph-vessels,  but  occur  also  in  bone- 
marrow,  lymphoid  tissue  and,  as  the  "wandering  cells,"  within  the  connec- 
tive and  epithelial  tissues.  Their  distribution,  therefore,  is  a  very  wide  one. 
When  examined  in  fresh  and  unstained  preparations,  the  colorless  cells 
or  leucocytes  appear  as  pale  nucleated  elements  which,  by  their  pearly  tint 
and   refracting   property,    are  readily   distinguished  from    the    much  more 

numerous  erythrocytes.  Their  shape 
is  variable,  but  when  first  withdrawn 
from  the  body  is  usually  irregularly 
spherical  or  oval.  When  placed  on 
a  warmed  slide  and  maintained  at 
the  temperature  of  the  body,  many, 
of  the  colorless  cells  exhibit  amoeboid 
motion,  w^hereby  not  only  alterations 
in  their  outlines  but  also  changes  in 
their  actual  position  are  produced. 
Although  always  present,  the  nucle- 
us may  be  obscured  by  the  overlying 
cytoplasm;  it  is  most  distinct  when 
the  cell  is  expanded,  as  when  under- 
going amoeboid  changes.  A  distinct 
cell-wall  is  absent,  although  probably 
the  most  superficial  zone  of  cyto- 
plasm possesses  slightly  greater  den- 
sity. The  size  of  the  colorless  corpuscles  varies  with  the  type  of  the  cell,  as 
described  below,  but  in  general  their  diameter  is  larger  than  that  of  the 
erythrocytes,  being  commonly  from  10-12  p..  Their  number  is  much  less 
than  that  of  the  red  cells,  the  usual  ratio  being'  about  one  colorless  to  six 
hundred  red  cells.  Even  within  physiological  limits  this  ratio  varies  con- 
siderably, from  5000  to  10,000,  with  an  average  of  7500,  white  cells  being 
normally  found  in  one  cubic  millimeter  of  human  blood. 

After  fixation  and  staining  (see  frontispiece),     five  varieties  of  colorless 
cells    may  usually  be  distinguished  in  normal   blood.'      Two  of  these  are 


Fig.  133. — Varieties  of  colorless  blood-cells  seen  in 
normal  human  blood  ;  a,  small  lymphocytes ;  b,  large 
lymphocyte  or  mononuclear  leucocyte ;  c,  trans- 
itional leucocyte;  rf.  polymorphonuclear  leucocytes  ; 
e,  eosinophile ;  y,  red  cells.    X  900. 


^  Tt  should  be  noted  that  the  differentiation  of  these  cells  is  founded  upon  not 
only  their  morphological  characters,  but  also  the  behavior  of  the  granules  embedded 
within  their  cytoplasm  when  subjected  to  certain  combination  stains.  A  generation 
ago  Ehrlich  divided  the  aniline  dyes  into  three  groups — acid,  basic,  and  neiiiral.  The 
first  includes  such  dyes  as  acid  fuchsin,  orange  G  or  eosin,  in  which  the  coloring  prin- 
ciple acts  or  exists  as  an  acid  and  exhibits  an  especial  affinity  for  the  cytoplasm. 
The  second  group,  the  basic  stains,  includes  dyes,  as  hematoxylin,  methylene-blue, 
methyl-violet,  methyl-green  or  thionin,  in  which  the  coloring  principle  exists  chemi- 
cally as  a  base  in  combination  with  a  colorless  acid  and  pariicularly  affects  the  chro- 
matin; hence  such  are  nuclear  stains.  Neutral  dyes,  produced  by  mixture  of 
solutions  of  an  acid  and  a  basic  stain,  have  a  selective  affinity  for  certain  so-called 
neutrophilic  granules. 


THE   COLORLESS    BLOOD-CELLS.  97 

known  as  lympJwcytes,  in  recognition  of  their  origin  from  lymphoid  tissue, 
and  the  others  as  leucocytes.  The  genetic  relations  of  these  groups  are  still  a 
subject  of  discussion.  According  to  one  view,  all  forms  of  colorless  cells  are 
derived  from  similar  sources,  the  recognized  varieties  being  only  different 
stages  in  the  development  of  the  same  elements.  The  other  view  regards 
the  leucocytes  and  the  lymphocytes  as  distinct  in  origin,  the  latter  arising 
within  lymphoid  tissue  and  the  former  chiefly  within  the  bone-marrow. 
A  discussion  of  this  mooted  subject  is  beyond  the  purpose  of  these  pages. 
Suffice  it  to  note,  that  facts  concerning  the  early  development  of  these  cells 
lend  support  to  the  belief  that  there  is  a  close  primary  relation  between  the 
two  groups. 

If  a  thin  film  of  blood  be  fixed  by  heat  and  stained  with  a  "triacid 
stain"  the  following  varieties  of  colorless  cells  are  distinguishable: 

1.  Small  Lymphocytes. — These  are  distinguished  by  a  large  com- 
pact deeply  staining  nucleus  that  occupies  almost  the  entire  cell.  The 
meagre  cytoplasm,  reduced  to  a  mere  narrow  peripheral  zone,  is  devoid  of 
granules.  The  small  lymphocytes  measure  about  7.5  //  in  diameter  and 
constitute  from  20-30  per  cent,  of  all  the  colorless  cells. 

2.  Large  Lymphocytes. — These  are  presumably  older  forms  of  the 
preceding  variety,  from  which  they  differ  in  their  larger  size  (12—15  //.) 
and  relatively  small  oval  nucleus.  The  cytoplasm  is  nongranular  and  com- 
parati\'ely  plentiful. 

3.  Large  Mononuclear  Leucocytes. — These  elements,  from  10- 
15  ij.  in  diameter,  exhibit  clear  pale  nuclei,  which  are  usually  eccentrically 
placed  and  oval  or  slightly  indented.  The  cytoplasm,  generous  in  amount, 
appears  uniform  save  for  tine  neutrophilic  granules  which  are  often  present. 

4.  Polymorphonuclear  Leucocytes. — These  are  by  far  the  most 
common  type  of  white  cells,  of  which  they  constitute  approximately  70  per 
cent.  Their  diameter  is  about  10  /^.,  hence  they  are  somewhat  larger  than 
the  erythrocytes.  The  cytoplasm  is  relatively  abundant  and  contains  fine 
neutrophilic  granules.  The  nuclei  are  very  conspicuous  on  account  of  their 
great  diversity  of  form.  At  first  sight  they  appear  multiple,  but  on  closer 
examination  are  seen  to  consist  of  irregular  nuclear  segments  connected 
by  delicate  processes.  Occasionally,  however,  two  or  more  actually  isolated, 
nuclei  exist,  in  such  cases  the  cells  being  truly  polynuclear. 

5.  Eosinophiles. — The  blood-cells  of  this  type  resemble  the  poly- 
morphonuclear leucocytes  in  size  (10  ,a)  and  in  the  lobulated  form  of  their 
nuclei.  Their  distinguishing  feature  is  the  presence  of  coarse,  highly 
refracting  granules  within  the  cytoplasm  that  display  an  especial  affinity  for 
eosin  and  other  acid  dyes.  The  eosinophiles  are  prone  to  rupture,  the  pale 
nucleus  then  lying  in  the  midst  of  a  swarm  of  brightly  tinged  granules. 

Mast-cells,  with  coarse  basophilic  granules,  and  the  finelv  granular 
basophiles  are  other  granular  colorless  cells  that  are  occasionally  observed,  as 
are  also  the  myelocytes  derived  from  the  bone-marrow.  They  are,  however, 
rarely  present  in  normal  blood. 

The  Blood-Plates. — Li  addition  to  the  erythrocytes  and  colorless 
cells,  the  blood  of  man,  and  perhaps  of  other  mammals,  constantly  contains 
small  bodies,  the  blood-plates  or  plaques.  They  are  exceedingly  prone  to 
change,  or  indeed  to  disappear  altogether,  when  exposed  to  the  air;  hence, 
to  insure  their  presence  in  an  unaltered  condition,  the  blood  must  be  drawn 
directly  into  a  drop  of  .8  per  cent,  salt  solution,  or,  still  better,  into  one  of 
osmic  acid  solution.  After  such  precautions,  the  blood-plates  appear  as 
round  or  oval  disks,  from  2-4  p.  in  diameter,  commonly  somewhat  less  than 
7 


98  NORMAL   HISTOLOGY. 

one  third  the  size  of  the  red  cells.  They  are  homogeneous  or  faintly  gran- 
ular, devoid  of  hemoglobin  and  nuclei,  and  never  exhibit  amoeboid  move- 
ment. They  may  be  directly  observed  as  free  bodies  circulating  within  the 
blood-vessels.  On  withdrawal  from  the  latter,  without  precautions  for  their 
preservation,  the  blood-plates  collect  in  irregular  masses  and  undergo  disin- 
tegration, their  remains  often  being  centres  from  which  radiate  the  threads 
of  fibrin.      The  significance  and  source  of  the  blood-plates  are  still  uncertain. 

They  have  been  variously  attributed  to 
disintegration  of  the  leucocytes,  to  ex- 
^^  trusion  from  the  red  cells,  to  destruction 

of  the  endothelium  of  the  vessels,  and, 
recently  by  Wright,  to  fragmentation  of 
long  processes  sent  out  by  the  giant  cells 
{megakaryocytes)  of  the  bone-marrow. 
In  view  of  their  constant  presence 
and  large  normal  quota — an  average  of 
*"  _  ^«!  400,000  or  more  in  one  cubic  millimeter  of 

^     *"    J  ®  blood — none  of  these  suggested  sources 

of  the  blood-plates  seems  satisfactory. 
^  Granules. — In  addition  to  the  cor- 

puscles   and    plates,    extremely    minute 

^"'-  '^cy^s"anTbioX^atls.°'^x  625?'^'^''°'    grauuks    occur    in  varying   numbers    in 

normal  blood.  The  nature  of  these  par- 
ticles differs.  Some,  but  probably  not  in  human  blood,  are  finely  divided 
fat;  others,  known  as  Jieniatoco7iia,  are  of  uncertain  composition  but  are 
not  fatty;  while  still  others  are  probably  derived  from  the  disintegration 
of  the  endothelial  and  blood-cells.  The  destruction  of  the  latter  accounts 
for  the  constant  presence  of  minute  particles  of  pigment. 

Blood-Crystals. — The  chief  constituent  of  the  red  cells,  the  .hemo- 
globin, probably  exists  within  the  corpuscles  as  an  amorphous  mass  in  com- 
bination with  other  substances,  from  which  it  must  be  freed  by  solution 
before  crystallization  occurs.  After  solution,  or  "laking, "  as  it  is  termed, 
the  coloring  matter  of  the  blood,  in  the  form  of  oxyhemoglobin,  separates 
into  microscopic  crystals,  usually  elongated  rhombic  or  rectangular  plates. 
On  mixing  dried  blood  with  a  few  grains  of  sodium  chloride  and  a  small 
quantity  of  glacial  acetic  acid,  and  heating  until  bubbles  appear,  minute 
brown  crystals  are  formed  in  large  numbers.  These  are  heniin  crystals  and 
derived  from  the  reduction  of  hemoglobin.  They  indicate  only  the 
presence  of  blood  and  are  valueless  in  differentiating  the  blood  of  man  from 
that  of  other  animals.  In  blood-clots  of  long  standing,  minute,  crystals  of 
hematoidin  often  appear  as  yellowish-red  plates.  This  substance  is  likewise 
a  reduction-product  of  hemoglobin. 

After  death,  or  upon  standing  after  withdrawal  from  the  body,  blood 
undergoes  coagulation,  whereby  the  corpuscles  become  entangled  among  the 
innumerable  delicate  filaments  of  fibrin.  In  microscopical  preparations  of 
fresh  blood,  the  fibrin  appears  after  a  time  within  the  plasma  in  the  form 
of  innumerable  delicate  threads,  which  cross  and  interlace  in  all  directions 
and  radiate  from  centres  marked  by  groups  of  blood-plates.  The  entangle- 
ment of  the  corpuscles  in  the  fibrin-net  results  in  the  production  of  a  dark- 
red,  jelly-like  mass,  the  blood-clot  or  crassamentum,  that  separates  from  the 
surrounding  clear  straw-colored  fluid,  the  serum.  In  stained  sections,  the 
white  cells  within  the  clot  are  readily  identified  as  deeply  tinted  bodies, 
particularly  along  the  free  border. 


ORIGIN   OF   VASCULAR   TISSUES.  99 


DEVELOPMENT  OF  THE  BLOOD-VASCULAR  TISSUES. 

The  Blood-Vessels. — The  earliest  blood-vessels  within  the  embryo 
are  networks  of  delicate  channels  within  the  mesoderm.  The  large  vessels 
of  the  trunk  arise  by  consolidation  and  fusion  of  the  axial  portions  of  the 
network;  the  extension  of  the  smaller  vessels  occurs  by  the  growth  and 
conversion  of  the  mesodermic  cells  with  which  the  primary  blood-tubes  are 
intimately  connected.  The  development  of  new  vessels  proceeds  from  the 
cells  constituting  the  walls  of  the  preexisting  channels.  These  walls  consist 
of  delicate  endothelial  plates  from  which  pointed  sprouts  grow  into  the 
surrounding  tissue.  These  outgrowths  are  at  first  solid,  but  later  become 
hollowed  out  by  the  gradual  extension  of  the  lumen  of  the  parent  vessel. 


Fig.  135. — Developing:  blood-vessels  in  embr>-onal  subcutaneous  tissue ;   a,  large  capillary ;  d,  young 
capillaries  ;  c,  solid  protoplasmic  outgrowths  forming  new  vessels.     X  300. 

All  vessels  consist  at  first  of  a  single  layer  of  endothelial  cells.  This  sim- 
plicity persists  in  the  capillaries,  while  the  walls  of  the  larger  vessels  become 
reinforced  by  additional  layers  differentiated  from  the  surrounding  mesoder- 
mic tissue. 

The  Erythrocytes. — The  first,  and  for  a  time  the  only,  blood-cells 
within  the  embryo  are  the  primary  erythroblasts  derived  from  the  meso- 
dermic elements  within  the  angioblastic  areas,  the  blood-islands.  These  cells, 
separated  by  the  colorless  plasma  which  appears  between  them  and  in  which 
they  henceforth  float,  undergo  mitotic  division  and  produce  nucleated  ele- 
ments that,  in  turn,  give  rise  to  similar  corpuscles.  The  earliest  erythroblasts 
are  relatively  large  round  nucleated  cells,  whose  cytoplasm  is  faintly  granular 
or,  possibly,  reticulated.  Their  large  nuclei  contain  networks  of  chromatin. 
For  a  time  the  granular  cytoplasm  is  colorless,  but  soon  becomes  deeply 
tinged  and  homogeneous  in  consequence  of  the  appearance  of  hemoglobin. 
The  primary  erythroblasts,  also  called  megaloblasts,  are  succeeded  by 
smaller  nucleated  cells,  the  secondary  erythroblasts  or  normoblasts,  which  are 
formed  chiefly  within  the  capillaries  (and  possibly  surrounding  tissues)  of  the 
liver  and,  probably,  to  a  limited  extent  within  the  spleen.  The  nuclei  of  the 
normoblasts  are  not  only  of  smaller  size  than  those  of  the  primary  blood- 
cells,  but  denser  and  much  more  compact.  By  mitotic  division,  the  normo- 
blasts give  rise  to  the  young  nucleated  erythrocytes,  which  lose  their  nuclei 


lOO 


NORMAL    HISTOLOGY. 


K^. 


\i    ^K.^^- 


FiG.  136. — Embryonal  blood  ;  the 
dividing  erytliroblasts  are  producing 
nucleated  erythrocytes.     X  600. 


and  become  the  ordinary  erythrocytes  or  red  blood-cells.  This  loss  of  the 
nuclei,  effected  probably  by  fragmentation  and  absorption  and  not  by  extru- 
sion, begins  in  the  human  embryo  about  the  second  month,  but  is  not 
completed  until  towards  the  close  of  foetal  life.  Even  at,  or  for  a  few- 
weeks  after  birth,  occasional  nucleated  erythrocytes  may  be  encountered  in 

the    circulation.      During    the   last   months  of 
foetal   life,   the    erythroblasts   retire   more   and 
■^    ^  more  within  the  red  bone-marrow,  which,  after 

v,_     ^  -  birth,  becomes  the  chief,  if  indeed  under  normal 

'  ®  conditions  not  the  exclusive,   seat  of  the  pro- 

duction of  new  red-cells  during  life.  Hence 
the  significance  and  frequent  classification  of  this 
tissue  (page  40)  as  a  blood-forming  organ. 

The  Colorless  Cells. — Our  knowledge 
concerning  the  origin  of  the  earliest  white 
blood-cells  is  incomplete  and  the  views  concern- 
ing the  genetic  relations  of  the  lymphocytes  and 
the  leucocytes  are  far  from  accord.  Although 
the  exact  location  is  uncertain,  it  is  generally 
assumed  that  these  cells  arise  from  mesodermic 
elements,  and  to  that  extent,  at  least,  share 
a  common  origin  with  the  erythrocytes.  Further,  that  the  white  cells  are 
formed  outside  the  blood-vascular  system,  which  they  subsequently  enter. 
The  assumption,  that  the  first  lymphocytes  are  formed  m  loco  within  the 
early  thymus  body,  by  the  metamorphosis  of  the  entodermic  epithelium,  and 
that  the  subsequent  migration  of  lymphocytes  so  derived  establishes  foci 
from  which  are  developed 

the   various    masses    of    lym-  Megakaryocyte 

phoid  tissue  throughout  the 
body,  needs  confirmation. 
The  progenitors  of  the 
leucocyte-group  of  color- 
less cells  seem  to  be  large 
elements,  the  Tnyeloblasts, 
which  appear  in  the  devel- 
oping liver  and  the  early 
bone-marrow  and  possess 
abundant  cytoplasm  devoid 
of  granules.  These  prima- 
ry elements  give  rise  to  the 
myelocytes,  which  exhibit  a 
granular  cytoplasm  and  are 
found  chiefly  in  the  bone- 
marrow  and,  to  a  limited 
extent,  the  spleen.  From 
the  myelocytes  descend  the 

various  forms  of  the  leucocytes  and,  probably,  the  huge  mononuclear 
marrow-cells,  the  megakaryocytes.  The  lymphocytes,  on  the  other  hand, 
are  the  especial  derivatives  of  the  lymphoid  tissues,  within  the  so-called 
germ-centres  in  which  they  arise  by  mitotic  division.  The  lymph-nodes, 
the  spleen  and  the  red  bone-marrow  are,  therefore,  the  most  important  seats 
of  the  production  of  the  colorless  blood-cells.  From  the  standpoint  of 
early  development,  the  sharp  distinction  between  the  lymphocytes  and  the 


Fig.  137. — Section  of  embryonal  bone-marrow,  showing  nucleated 
erythrocytes,  leucocytes  and  megakaryocyte.     X  625. 


THE    HEART.  loi 

leucocytes,  as  insisted  on  by  Ehrlich  and  his  supporters,  based  on  the 
assumption  that  the  leucocytes  originate  exclusi\ely  within  the  bone- 
marrow,  is  open  to  challenge.  As  shown  by  Ebner,  all  the  typical  forms 
of  white  cells,  including  the  polymorphonuclear  leucocytes,  appear  before 
the  differentiation  of  the  earliest  bone-marrow.  It  is  not  improbable,  there- 
fore, that  all  forms  of  the  white  cells  are  related  genetically  and  traceable  to 
common  ancestors. 

THE  HEART. 

In  principle,  the  heart  is  a  modified  blood-vessel,  formed  by  the  fusion 
of  two  heart-tubes  and  converted  into  an  efficient  organ  for  propulsion  by  the 
unusual  development  of  muscular  tissue  within  its  walls.  As  are  the  walls 
of  the  larger  arteries,  so  also  is  that  of  the  heart  composed  of  three  general 
layers.  The  inner  of  these,  the  endocardium,  consists  of  an  endothelial  lining 
and  fibro-elastic  tissue.  The  middle  layer,  the  myocardium,  contributes  by 
far  the  greatest  bulk  of  heart-tissue  and  is  made  up  of  intricately  arranged 


Deepest 
laver 


Heart  muscle  %.".*'    '      j?  Z^     -i  .5^=1    i,""  ^  •-  '->•- 


Blood-vessel  -'~  ""^^n^^  \  V^\ '"^ 


sheets  of  cardiac  muscle  and  fibro-elastic  tissue.  The  outer  layer,  the  epicar- 
dium,  the  visceral  layer  of  the  pericardium,  is  a  stratum  of  fibro-elastic  tissue, 
covered  externally,  except  at  the  base  where  the  great  vessels  join  the  heart, 
with  endothelium. 

The  Endocardium. — The  endocardium  follows  all  the  irregularities 
of  the  interior  of  the  heart,  lining-  every  recess  and  covering  the  free  surfaces 
of  the  valves,  tendinous  cords  and  papillary  muscles.  It  consists  of  a  single 
layer  of  endothelial  plates  and  the  underlying  connective  tissue.  The  latter 
contains  scattered  strands  of  involuntary  muscle  and  is  rich  in  elastic  fibres. 
The  elastic  tissue  occupies  particularly  the  deeper  parts  of  the  endocardium, 
being  almost  wanting  beneath  the  endothelium,  and  in  the  auricles  or  atria, 
where  it  is  most  abundant,  may  be  condensed  into  fenestrated  membranes. 
The  deepest  layer  of  the  endocardium  blends  with  the  connective  tissue  of 
the  subjacent  myocardium. 

The  valves  of  the  heart  are  essentially  duplicatures  of  the  endocardium, 
strengthened  in  their  thicker  parts  by  fibro-elastic  tissue.  In  the  case  of  the 
atrio-ventricular  valves,  this  tissue  is  continuous  with  the  dense  fibrous  rings 
(annuli  fibrosi)  to  which  the  leaflets  are  attached.  Towards  the  free  margin 
of  the  valve,  the  layers  are  blended  and  reduced  to  a  thin  fibrous  stratum 
covered  on  both  sides  by  endothelium.      Strands  of  nonstriated  muscle  occur 


I02 


NORMAL   HISTOLOGY. 


Muscular  tissue 


Fibro- 
elastic 
tissue  of 
valve- 
leaflef 


near  their  attached  borders,  while  the  fibro-elastic  tissue  of  the  chorda  ten- 
dinese  is  continuous  with  the  middle  layer.  The  semihmar  valves  guarding 
the  aorta  and  the  pulmonary  artery  correspond  in  their  general  structure 
with  the  other  valves,  although  muscle  in  them  is  wanting.  At  the  periphery 
and  in  the  central  thickenings  or  noduli  of  the  leaflets,  the  elastic  tissue  is 
particularly  rich.  Within  the  folds  guarding  the  orifices  of  the  inferior  vena 
cava  and  of  the  coronary  sinus,  the  interendothelial  connective  tissue  is  an 
inconsiderable  layer,  which,  in  the  case  of  the  Eustachian  valve,  is  sometimes 
further  reduced  by  absorption  resulting  in  a  fenestrated  condition  of  the  leaflet. 
The  Myocardium. — The  middle  layer  of  the  heart- wall,  the  myocar- 
dium, is  composed  of  a  close  elongated  network  of  muscle-fibres,  the  inter- 
muscular spaces  of  which  are  filled  with  connective  tissue.  The  latter 
corresponds  to  an  endomysium  and,  within  the  ventricles,  contains  only 
a  small  amount  of  elastic  tissue,  except  around  the  openings  of  the  valves. 

In  these  locations,  dense  plates 
of  fibrous  tissue  (annulifibrosi) 
encircle  the  valvular  orifices 
and  contain  many  elastic  fibres, 
as  well  as  give  attachment 
to  the  strands  of  cardiac  mus- 
cle. The  histology  of  cardiac 
muscle  has  been  described 
elsewhere  (page  55)  and 
need  not  here  be  considered. 
Although  as  ordinarily  seen 
in  microscopical  preparations, 
the  fibro-muscular  sheets  that 
compose  the  myocardium  seem 
to  follow  no  particular  airrange- 
ment,  it  has  been  shown  that 
in  the  architecture  of  the  heart' 
they  are  disposed  according 
to  a  definite  but  complex 
plan.  For  a  description  of 
this  arrangement,  however,  the 
reader  must  be  referred  to 
the  systematic  text-books  of 
anatomy. 

In  addition  to  the  ordi- 
nary fibres  of  cardiac  muscle, 
the  subendocardial  layer  of  the  myocardium  in  many  places,  especially  in  the 
ventricles,  contains  peculiar  fibres,  distinguished  by  their  large  size,  pale  color 
and  abundant  sarcoplasm,  with  a  corresponding  lessening  in  the  number  of 
contractile  fibrillae.  These  have  long  been  known  as  Purkinje  fibres  and 
were  regarded  as  immature  and  imperfecdy  differentiated  muscle-elements. 
Their  significance,  however,  has  only  recendy  been  recognized.  ^  They  are 
now  regarded  as  the  terminal  part  of  an  elaborate  system  of  special  muscle- 
fibres,  whose  probable  function  is  the  coordinative  connecdon  of  the  auricular 
and  ventricular  musculature,  that  otherwise  are  distinct  and  unconnected. 
The  most  evident  part  of  this  system  is  the  definite  atrio-ventriadar  bundle, 
which,  beginning  in  the  auricular  wall  in  the  vicinity  of  the  coronary- sinus, 
passes  from  the  auricular  septum,  over  the  attachment  of  the  posterior 
leaflet  of  the  tricuspid  valve,  into  the  pars  membranacea  septi;  here  dividing 


Fig. 


139.— Longitudinal  section   of  leaflet  of  tricuspid 
valve.    X  20. 


THE   HEART.  103 

into  a  right  and  left  limb,  the  bundle  continues  into  the  interventricular 
partition.  Although  distinct  and  compact  during  this  course,  at  its  two 
ends  the  atrio-ventricular  bundle  breaks  up  into  radiating  and  interlacing 
strands,  which  form  an  intricate  network  composed  of  Purkinje  fibres.  The 
latter  disappear  among  the  elements  of  the  myocardium  by  gradual  transition 
into  the  ordinary  fibres  of  cardiac  muscle.  The  Purkinje  fibres  ramify  not 
only  within  the  main  walls  of  the  heart-chambers,  but  also  invade  the  tra- 
becula  (columnar  carneae)  and  papillary  muscles  of  the  ventricles.  For  a 
time  muscular  throughout  their  length,  the  papillary  muscles  become  trans- 
formed into  the  fibrous  chordae  tendinese  in  the  segments  attached  to  the 
valves.  These  fibrous  cords  often  contain  considerable  elastic  tissue  which 
is  continuous  with  the  fibro-elastic  layer  of  the  valve-leaflets. 

The  Epicardium. — The  external  layer  of  the  heart-wall,  the  epicar- 
dium,  corresponds  in  its  general  structure  with  other  parts  of  the  pericar- 
dium. It  consists,  as  do  other  serous  membranes,  of  a  single  layer  of  endo- 
thelial cells  that  covers  the  free  surface  of  the  heart  and  rests  upon  a  stratum 
of  fibro-elastic  connective  tissue.  The  elastic  fibres  are  very  fine  and 
numerous  and  form  a  dense  network  immediately  beneath  the  endothelium. 
Those  within  the  auricular  epicardium  are  prolonged  into  the  adventitia  of 
the  great  veins,  while  the  elastic  fibres  of  the  ventricular  covering  end  before 
reaching  the  aorta  and  pulmonary  artery.  Where  not  separated  from  the 
muscle  by  subserous  fat,  which  may  be  abundantly  present  even  in  normal 
hearts,  especially  in  the  auriculo-ventricular  and  the  interventricular 
grooves,  the  epicardium  is  intimately  attached  to  the  subjacent  muscular 
coat.  The  numerous  branches  of  the  coronary  vessels,  as  well  as  the  nerve- 
trunks  and  the  microscopic  ganglia  connected  with  the  coronary  plexuses, 
lie  beneath  the  epicardium  or  within  its  deepest  layer. 

Blood-Vessels. — The  unusually  generous  vascular  supply  of  the  heart 
includes  the  branches  derived  from  the  coronary  arteries  and  the  capillaries. 
The  former  ramify  beneath  the  epicardium  and  are,  to  some  extent,  erid- 
arteries,  that  is,  arteries  which  do  not  directly  anastomose  with  their  neighbors. 
Although  both  the  epicardium  and  the  deepest  layer  of  the  endocardium 
contain  small  vessels  destined  for  their  tissues,  it  is  to  the  heart-muscle  that 
the  blood  is  chiefly  directed.  The  larger  vessels  of  the  myocardium  course 
within  the  more  robust  tracts  of  connective  tissue,  giving  off  the  twigs  which 
resolve  into  the  capillary  networks.  These  exhibit  elongated  meshes,  similar 
to  those  seen  in  voluntary  muscle,  which  enclose  the  muscle-fibres.  The  re- 
lation of  the  capillaries  to  the  individual  fibres  is  most  intimate,  since  in  many 
places  the  capillaries  are  received  in  grooves,  or  almost  tunnels,  in  the  muscle- 
substance.  The  valves  are  devoid  of  blood-vessels,  with  the  exception  of 
those  accompanying  muscular  tissue  within  the  bases  of  the  auriculo-ventric- 
ular leaflets. 

The  lymphatics  of  the  heart  are  represented  by  the  numerous  lymph- 
spaces  within  the  connective  tissue  between  the  muscle-bundles,  and  by  the 
more  definite  lymphatic  vessels.  The  latter  form  two  sets,  a  network  within 
the  deepest  layer  of  the  endocardium  and  a  network  beneath  or  within  the 
epicardium.  These  networks  communicate  with  the  larger  lymphatic  vessels 
which  lie  in  the  auriculo-ventricular  groove. 

The  nerves  are  many  and  contributed  by  the  vagus  and  the  sympathetic. 
They  include  both  medullated  and  nonmeduUated  fibres  which  form  the  cor- 
onary and  many  small  subsidiary  plexuses.  Scattered  in  these  superficial 
plexuses  lie  numerous  nerve-cells,  sometimes  singly  but  often  collected  into 
microscopic  ganglia.      They  are  especially  plentiful  around  the  orifices  of  the 


I04  NORMAL   HISTOLOGY. 

great  veins  opening  into  the  auricles,  a  vicinity  corresponding  to  the  upper 
end  of  the  atrio-ventricular  bundles,  and  over  the  upper  parts  of  the  ventricles. 
Nerve-fibres  and  ganglion-cells  have  been  demonstrated  within  the  atrio- 
ventricular bundle.  The  distribution-twigs  contain  both  efferent  (motor) 
and  afferent  (sensory)  fibres.  The  immediate  motor  fibres  supplying  the 
heart-muscle,  whose  axis-cylinders  end  on  the  surface  of  the  muscle-fibres 
usually  in  minute  swellings  (page  86),  are  probably  exclusively  the  axones  of 
sympathetic  neurones,  since  it  is  doubtful  whether  the  vagus  fibres  extend  be- 
yond the  cell-bodies  of  such  neurones,  which  they  enclose  in  terminal  arbor- 
izations. The  sensory  fibres,  at  least  in  part  from  the  vagus,  are  distributed 
to  the  epicardium,  endocardium  and  the  connective  tissue  of  the  myocardium. 
Within  the  epicardium  especially  and  to  a  limited  degree  also  in  the  other 
layers,  the  afferent  fibres  are  connected  with  endings  which  resemble  the  neu- 
rotendinous spindles. 

The  Pericardium. — The  parietal  pericardium  corresponds  in  its  gen- 
eral structure  to  that  of  the  visceral  portion,  the  epicardium,  above  described. 
Its  free  surface  is  covered  with  a  single  layer  of  endothelial  plates,  which  rest 
on  the  connective  tissue  layer.  The  latter  consists  of  fairly  dense  fibrous 
tissue,  intermingled  with  fine  elastic  fibres,  which  form  a  close  network  imme- 
diately beneath  the  endothelium.  Where  not  intimately  attached  to  the 
pleura,  a  much  looser  subserous  layer  of  fibro-elastic  connective  tissue  is 
present.  This,  as  well  as  the  outer  part  of  the  pericardium,  contains  a  vari- 
able amount  of  adipose  tissue.  The  blood-vessels  and  nerves  of  the  pericar- 
dium are  comparatively  few  ;  some  of  the  nerves,  which  are  chiefly  afferent, 
are  connected  with  Pacinian  corpuscles.  The  lymphatics  are  represented  by 
interfascicular  lymph-spaces  and  more  definite  channels  within  the  connective 
tissue.  The  lymphatics  beneath  the  endothelium  possess  thin  walls  and  stand 
in  intimate  relation  to  the  pericardial  sac,  particles  passing  between  the  endo- 
thelial plates  into,  the  lymph-channels  although  no  preformed  openings  or 
stomata  exist. 

Development  of  the  Heart. — A  systematic  account  of  the  formation 
of  the  heart  is  beyond  the  purpose  of  these  pages.  Suffice  it  to  note,  that, 
very  early  in  the  young  embryo,  two  heart-tubes  are  folded  of?  in  the  visceral 
layer  of  the  mesoderm.  These  tubes,  at  first  entirely  separate,  gradually 
approach  the  ventral  mid-line  and  eventually  fuse,  a  single  heart  thereby 
arising.  The  wall  of  the  latter,  as  well  as  of  the  primary  tubes,  consists  of 
two  layers,  separated  by  a  distinct  space.  The  inner  of  these  layers  is  com- 
posed of  a  single  strand  of  very  delicate  mesodermic  (mesenchymal)  ele- 
ments, which  become  the  lining  of  the  heart;  it  is,  therefore,  known  as  the 
endothelial  heart  and  lies  within  the  outer  myocardial  layer  of  the  heart-wall 
as  a  shrunken  cast  within  a  mould.  The  outer  layer  is,  from  the  first, 
thicker  and  exhibits  a  tendency  to  form  trabeculae,  the  resulting  myocardium 
being  for  a  time  loose  and  spongy.  Later,  the  two  layers  of  the  heart-wall 
come  into  contact,  when  the  endothelial  stratum  becomes  applied  to  the 
irregular  surface  of  the  myocardium,  every  ridge  and  pocket  of  which 
receives  an  investment  of  endothelium.  The  subsequent  consolidation 
which  the  heart-walls  undergo  brings  about  the  effacement  of  the  primary 
spongy  texture  of  the  myocardium,  except  within  the  innermost  zone,  where 
the  ridges  and  bands  of  the  columnae  carneae  remain  throughout  life  as  the 
manifestations  of  the  primary  condition.  During  these  changes,  the  mesen- 
chyma  of  the  trabecul£e  differentiates  into  the  syncytium,  from  which  arise 
the  myofibrils  of  the  later  heart-muscle  (page  6i),  and  into  the  connective 
tissue  filling-  the    interstices  between   the    network  of    muscle-fibres.      The 


THE   LYMPHATIC   SYSTEM. 


105 


Mesocardium 


Structural  peculiarities  of  the  cardiac  fibres,  as  contrasted  with  those  of 
ordinary  striated  muscles,  indicate  a  less  complete  differentiation  in  the  heart- 
muscle,  this  being  particularly  true  of  the  Purkinje  fibres.  These  charac- 
teristics are  probably  ccfrrelated  with  the  exceptional  activity  that  the 
heart-muscle  is  called  upon  to 
endure,  since,  as  seen  in  the 
"red"  muscles  (page  59),  a 
lower  degree  of  histological 
differentiation  favors  pro- 
longed exertion,  although  at 
the  expense  of  rapidity  of 
contraction.  The  valves  are 
formed  from  cushion-like 
thickenings  of  the  mesenchy- 
ma.  Those  surrounding  the 
primary  efferent  vessel,  the 
truncus  arteriosus,  lead  to 
the  subdivision  of  this  tube 
into  the  aorta  and  the  pul- 
monary artery  and,  likewise, 
to  the  formation  of  the  three 
leaflets  of  the  semilunar 
valves.  In  the  case  of  the 
auriculo-ventricular  valves, 
the  septal  leaflets  are  formed 
from  the  endocardial  cush- 
ions, which  appear  on  the 
surfaces  of  the  partition  (sep- 
tum intermedium)  that  divides  the  auricular  canal,  the  channel  connecting  the 
primapy  auricular  and  ventricular  segments  of  the  heart.  The  other  leaflets 
of  these  valves  are  derived  from  the  walls  of  the  auricular  canal,  a  process  of 
undermining  partially  freeing  portions  of  the  innermost  layer  of  the  heart- 
wall.  These  overhanging  plates  are  connected,  however,  with  the  ven- 
tricular myocardium  by  strands  of  tissue,  the  later  papillary  muscles.  The 
latter  for  a  time  are  entirely  muscular,  but  later  the  muscle-tissue  disappears 
near  the  valve-leaflet  and  the  bands  are  converted  into  the  fibrous  strands, 
the  chordae  tendineae. ' 


Enrlothelial 
la\er 

//[ 

Myocardial 
layer 

\  Primitive  ventricle 

Fig.  140. — Transverse  section  of  early  rabbit  embryo  pass- 
ing through  young  heart,  showing  venous  segment  behind 
and  arterial  in  front.     ,\  75. 


THE   LYMPHATIC  SYSTEM. 

The  lymphatic  or  lymph-vascular  system  consists  of  an  almost  uni- 
versally present  system  of  channels,  some  of  which  are  definite  tubes,  the 
lymphatic  vessels,  and  others  uncertain  and  often  illy  defined  clefts,  the 
lymph-spaces,  between  the  bundles  of  connective  tissue.  The  vessels  con- 
tain the  lymph,  a  fluid  usually  colorless  and  containing  numerous  corpuscles, 
the  lymphocytes.  Since  the  latter  are  familiar  as  one  of  the  chief  types  of 
colorless  blood-cells,  they  are  described  in  connection  with  the  blood  (page 
96),  in  which  they  circulate.  Although  the  lymph  is  ordinarily  clear,  that 
within  the  lymphatics  leading  from  the  intestines  appears,  especially  during 
digestion,  more  or  less  milky,  in  consequence  of  the  lymph-cells  being 
loaded  with  particles  of  fat  which  they  have  taken  up  from  the  intestinal 
contents.  For  this  reason  these  intestinal  lymphatics  are  often  termed 
lacteals.      The  lymphatics   resemble   the  veins,    from   which,    indeed,    they 


io6 


NORMAL  HISTOLOGY. 


probably  originate,  and  into  which  they  finally  pour  their  contents.      They 
arise  from  capillaries,  have  walls  closely  resembling  in  structure  those  of  the 


Valve 


Lymph-vessel 


Lymph- 
space 


Deeply  stained 

^;round- 

substance 


Fig.  141.— Portion  of  central  tendon  of  rabbit's  diaphragm,  treated  with  silver  nitrate  ;  lymphatic  vessels 
are  shown  as  light  irregular  tracts  ;  lymph-spaces  are  seen  within  stained  ground-substance.     X  120. 


veins,  and  are  provided  with  many  valves. 


VT^ 


;r 


M^-! 


On  the  other  hand,  the  lym- 
phatics form  a  system 
which  is  closed,  except 
where  the  two  chief  trunks 
open  into  the  subclavian 
veins,  the  capillaries  be- 
ginning as  blind  channels. 
The  most  striking  feature 
of  the  lymph-paths,  how- 
ever, is  the  presence  along 
the  vessels  of  more  or 
less  conspicuous  masses 
of  lymphoid  tissue,  the 
lymph-nodes,  often  mis- 
called lymphatic  '  'glands' ' 
( lympho-glandulee). 

Lymph-Spaces.  — 
These  spaces  exist  practi- 
cally in  all  structures  of 
the  body,  for  the  most 
part,  however,  as  the  in- 
terfascicular clefts  within 
connective  tissue.  The 
lymph-spaces  are  filled  by  a  clear  watery  fluid,  the  tissue-juices,  and  are 
imperfectly  lined  by  flattened  connective  tissue  cells.      The  spaces  present 


vVV\-yy-" 


Fig.   142.- 


-Perivascuiar  lymph-spaces  sunoiindinr 
vessels.     >(  225. 


retinal    blood 


THE   LYMPH-VESSELS. 


107 


great  \'ariations  in  size.  In  some  localities,  as  within  the  nervous  tissues,  they 
surround  even  individual  cells;  in  other  places  they  are  represented,  in  princi- 
ple at  least,  by  large  cavities,  since  the  subdural  and  subarachnoid  spaces,  the 
chambers  of  the  eye,  the  channels  of  the  internal  ear  occupied  by  the  peri- 
lymph,   the   synovial    sacs   of    the 


\dveiititia 

Media 
ndothelium 

Lymph-cells 


Fig.  143. — Transverse  section  of  small  lymph- 
vessel.      ■    160. 


joints,  and,  indeed,  the  great  serous 
cavities — the  pericardial,  pleural  and 
peritoneal  sacs — are  all  regarded  as 
belonging  to  the  lymph-spaces. 
Although  a  conclusion  not  beyond 
discussion,  the  lymph-sacs  are  now 
believed  to  form  a  closed  system  of 
intercommunicating  channels,  which, 
while  in  intimate  relation  with  the 
lymphatic  capillaries,  do  not  actually 
open  into  the  latter.  In  many  local- 
ities, however,  the  spaces  and  capil- 
laries are  separated  by  only  delicate 
partitions  which  allow  the  passage 
of  fluids,  and  also  of  lymphocytes, 
from  the  tissue-spaces  into  the 
lymph-vessels.  Within  the  adventi- 
tious coat  of  certain  blood-vessels, 
conspicuously  those  of  the  retina, 
the  surrounding  lymphatic  channels  constitute  perivascular  lymph-spaces. 
The  Lymph- Vessels. — The  definite  lymph-paths  include  the  capilla- 
ries and  the  vessels.  The  lymphatic  capillaries  are  arranged  in  networks,  vary- 
ing in  closeness  and  complexity,  and  resemble  in  structure  the  blood-capil- 
laries, consisting  of  a  single  layer  of  endothelial  plates.      They  differ  from 

the  blood  capillaries  in  being  usu- 
ally much  greater  in  calibre  and  less 
regular  in  size  (30-60  /^.),  larger 
and  smaller  capillaries,  often  beset 
with  irregular  constructions  and 
enlargements,  being  indefinitely 
interspersed. 

The  more  formal  lymph-chan- 
nels, the  lymphatics ,  as  they  are 
commonly  called,  which  arise  from 
the  networks  of  lymph-capillaries 
and  convey  the  lymph  ultimately 
to  the  subclavian  veins,  closely 
resemble  the  veins  in  arrangement 
and  structure.  The  larger  lymph- 
vessels  (from  .5  mm.  and  upwards) 
possess  walls  consisting  of  three 
coats,  which  are  much  like  those 
of  the  veins.  These  include:  («) 
the  intima,  composed  of  the  endothelial  lining  and  a  thin  layer  of  fibro- 
elastic  tissue;  {b)  the  media,  made  up  of  circular  involuntary  muscle  inter- 
spersed with  connective  tissue  and  few  elastic  fibres;  and  (r)  the  adventitia, 
consisting  of  fibro-elastic  tissue  and,  sometimes,  of  longitudinal  bundles  of 
smooth  muscle.      The  numerous  valves  are  essentially  folds  of  the  intima. 


Leaflets  of 
valve 


Endothelium 


Fig.  144.— Section  of  lymphalic.  showing  valve.    X  iSo. 


io8 


NORMAL   HISTOLOGY. 


Lymphoid  Tissue. — Wherever  found,  whether  as  diffuse  masses, 
simple  nodules,  or  as  the  larger  and  complex  lymph-nodes,  lymphoid  or 
adenoid  tissue  is  composed  of  two  chief  constituents — the  supporting  con- 
nective tissue  i-eticuluni  and  the 
lymphoid  cells  contained  within 
the  meshes  of  the  reticulum. 
The  latter  varies  in  the  thickness 
of  the  component  fibres  and  the 
size  of  its  meshes,  but  in  the 
denser  types  of  lymphoid  tissue, 
as  in  the  periphery  of  the  solitary 
nodules  and  in  the  cortical  folli- 
cles and  medullary  cords  of  the 
lymph-nodes,  it  is  so  masked  by 
the  innumerable  overlying  cells 
that  only  after  removal  of  the 
latter  can  the  supporting  frame- 
work be  satisfactorily  demon- 
strated. The  reticulum  is  modi- 
fied connective  tissue  (page 
22),  upon  the  surface  of  whose 
trabeculae,  particularly  at  the 
points  of  juncture,  flattened  con- 
nective tissue  cells  are  closely 
applied.  Where  of  exceptional 
delicacy,  the  reticulum  is  formed 
almost  entirely  by  the  anasto- 
mosing processes  of  the  stellate 
connective  tissue  cells. .  The 
lymphoid  cells  are  exceedingly 
numerous  and  closely  packed  and  present  the  characteristics  of  the  lympho- 
cytes in  the  blood,  this  resemblance  being  explained  by  the  fact  that  such 
blood-cells  are  derived  from  the  lymphoid 
tissues. 

The  simple  lymph-nodules,  vary- 
ing in  size  but  seldom  more  than  2  mm. 
in  diameter,  are  irregularly  spherical  or 
ellipsoidal  masses  of  lymphoid  tissue,  in 
which  a  denser  peripheral  zone  encloses 
and  blends  with  a  less  compact  core. 
Within  the  latter,  which  being  of  looser 
texture  appears  as  a  lighter  central  area, 
usually  are  seen  lymphoid  cells  in  various 
stages  of  mitotic  division.  Such  foci  are 
known  as  germ-centres  and  indicate  the 
birthplaces  of  many  new  lymphocytes. 
Although  the  limits  of  the  lymph-nodules 
are  commonly  imperfectly  defined  by  a 
condensation  of  the  surrounding  connective 
tissue,  a  distinct  capsule  is  wanting.  Definite  lymph-channels  are  found 
neither  upon  the  surface  nor  within  the  simple  nodules;  the  latter  are 
provided,  however,  with  a  generous  network  of  capillary  blood-vessels. 
Intermediate  in  complexity,  between  the  simple  nodules  on   the  one  hand 


V\G.  145. — Simple  lymph-nodule  from  large  intestine. 
X  120. 


Fig.  146. — Portion  of  lymph-nodule  show- 
ing details  of  germ-centre.     X  280. 


THE   LYMPH-NODES. 


109 


Trabeculae 


and  the  typical  lymph-nodes  on  the  other,  stand  such  structures  as  Peyer's 
patches  and  the  faucial  tonsils,  in  which  groups  of  simple  nodules  are  blended 
into  a  single  organ,  the  component  nodules  only  partially  retaining  their 
individuality. 

The  lymph-nodes  are  flattened  oval  or  bean-shaped  bodies,  from  a  few 
millimeters  to  two  centimeters  or  over  in  length,  that  are  scattered  along  the 
lymphatic  vessels,  some- 
times singly  but  often  in 
chains  or  groups.  On 
nearing  a  node,  the  lymph- 
vessel  divides  into  a  num- 
ber of  stems,  the  afferent 
vessels,  which  enter  the 
substance  of  the  node  and 
communicate  with  the  cap- 
illary network  within  its 
interior.  From  the  latter 
other  channels,  the  efferent 
vessels,  arise  and  emerge 
from  the  node  at  a  point 
usually,  but  not  always, 
marked  by  a  slight  de- 
pression, the  hilum.  The 
lymph-nodes  are  invested 
by  a  distinct  fibrous  capszile, 
in  which  elastic  fibres  and  occasional  unstriped  muscle  are  present.  From 
the  inner  surface  of  this  envelope,  the  fibro-elastic  tissue  is  continued  into 
the  substance  of  the  node  in  the  form  of  numerous  radially  directed  trabeculae. 


Lymph- 

sitiusi 


Capsule 


Ftg.  147. — Diagram  illustrating  architecture  of  lymph-node. 


Germ-     P'*' 
Lymph-sinus    centre      ^si 


Lymph-sinus 


Lymph-sinus 


Capsule 
^Trabecula 

Cortical  follicles 


Hilum  Efferent  lymphatics    Medullary  cords 


Fig.  148. — Section  of  small  lymph-node  through  hilum.     X  23. 


which  thus  subdivide  the  other  zone,  or  cortex,  into  a  series  of  compartments. 
On  reaching  the  inner  limit  of  the  cortical  zone,  the  trabeculae  are  less 
regular  and  freely  anastomose,  thereby  breaking  up  the  deeper  parts  of  the 
node,   the  medulla,  into  uncertain  cylindrical   compartments.     The   spaces 


no  NORMAL   HISTOLOGY. 

thus  imperfectly  defined  by  the  trabeculae  are  incompletely  filled  by  masses 
of  compact  lymphoid  tissue,  the  general  form  and  arrangement  of  which 
correspond  to  the  compartments  in  which  they  lie.  The  masses  contained 
within  the  peripheral  spaces  are  irregularly  spherical  or  pyramidal  and 
constitute  the  cortical  nodules ;  those  within  the  intercommunicating  central 
compartments  form  a  network  of  irregular  cylinders,  the  medullary  cords, 
which  are  continuous  with  one  another  and  with  the  deeper  part  of  the 
cortical  nodules  (Fig.  148). 

The  intervals  between  the  tracts  of  lymphoid  tissue  and  the  trabecular 
framework  constitute  a  system  of  freely  communicating  channels,  the  lymph- 


Cortical  follicle  • 


Lymph-sinus 


Lymph-sinus  ■ 


Medullary  cord 


J^ 


Fig.  149. — Portion  of  periphery  of  lymph-node,  showing  relation  between  trabecula,  sinus,  and  lymphoid 

tissue.     X  50. 

sinuses,  through  which  slowly  passes  the  lymph  brought  to  the  node  by 
the  afferent  lymphatic  vessels.  The  latter  pierce  the  capsule  on  the  convex 
surface  of  the  node  and  open  into  the  sinuses  that  partially  surround  the 
cortical  nodules.  After  traversing  the  peripheral  sinuses,  the  lymph  passes 
into  the  irregular  channels  of  the  medulla  and  finally  escapes  from  the  node 
through  the  efferent  lymphatics,  which  usually  emerge  at  the  hilum,  if  one 
be  present,  on  the  surface  of  the  node  opposite  to  the  entrance  of  the  efferent 
vessels.  The  lymph-sinuses,  therefore,  are  bounded  on  one  side  by  the 
capsule  or  trabeculae  and  on  the  other  by  the  masses  of  dense  lymphoid- 
tissue.  The  lumen  of  these  channels,  however,  is  not  free,  but  occupied  by 
a  delicate  wide-meshed  reticulum  consisting  of  fine  strands  of  connective 
tissue  where  most  marked,  or  of  the  anastomosing  processes  of  stellate  cells 
where  very  delicate.  The  sinuses  are  lined  by  an  imperfect  layer  of  flattened 
plate-like  cells,  that  represent  the  endothelium  of  the  adjoining  lymphatic 


THE   LYMPH-NODES.  iii 

vessels.      Althoug-h  both  the  afferent  and  efferent  lymphatics  are  provided 
with  valves  close  to  the  node,  no  such  folds  occur  along  the  sinuses.      The 


Trabecula 


Lymph-sinus 


Fig.  150.— Portion  of  medulla  of  lymph-node,  showing  details  of  lymph-sinus  and  medullary 

cords.     X  250. 

passage  of  the  lymph  through  the  node  is  retarded  by  the  reticulum  within 
the  sinuses,  thus  favoring  the  entrance  of  the  young  lymphocytes  from  the 


Fig.  151. — Cross-section  of  small  lymph-node,  injected  to  show  rich  vascular  supply.    X  10. 

bordering  lymphoid  tissue  into  the  sluggishly  moving  lymph-stream.  Germ- 
centres,  the  particular  foci  for  the  production  of  lymphocytes,  usually  are 
present  within  the  cortical  nodules,  but  are  not  found  in  the  medullary  cords. 


112 


NORMAL    HISTOLOGY. 


Capsul 


Lymph 


The  blood-vessels  for  the  nutrition  of  the  lymph-nodes  are  numerous. 
Some  pierce  the  surface  of  the  node  at  various  points  and  are  distributed  to 
the  capsule  and  the  trabeculae;  most,  however,  enter  through  the  hilum. 
After  following  for  a  short  distance  the  trabeculae,  the  arterioles  cross  the 
sinuses  and  enter  the  cords  and  nodules  of  the  denser  lymphoid  tissue, 
within  which  they  break  up  into  rich  capillary  networks. 

The  nerves  enter  the  lymph-nodes  at  the  hiium,  in  company  with  the 
blood-vessels.  They  include  both  meduUated  and  nonmedullated  fibres,  but 
are  chiefly  sympathetic  fibres  destined  for  the  involuntary  muscle  of  the 
vessels  and  of  the  capsule. 

Hemolymph  Nodes. — In  addition  to  the  ordinary  lymph-nodes, 
there  occur  in  various  regions,  especially  in  the  prevertebral  region  of  the 

abdomen,  structures  which 
resemble  lymph-nodes  in 
form  and  size,  but  differ 
from  them  in  the  deep  red 
color  which  they  usually 
exhibit.  These  bodies  are 
known  as  the  hemolymph 
nodes.  Their  distinguish- 
ing feature  is  the  substitu- 
tion of  blood-channels  for 
the  usual  lymph-sinuses, 
which,  in  the  typical  hem- 
olymph nodes,  may  be  en- 
tirely wanting.  The  path 
of  the  blood  resembles  that 
within  the  spleen,  since  the 
blood-cells  escape  from  the 
imperfectly  walled  vessels 
into  the  lymphoid  tissue 
and  thence  pass  into  the 
blood -sinuses  and  on  to 
the  veins.  In  many  cases, 
however,  the  substitution 
of  the  lymph-sinuses  by 
blood-spaces  is  not  com- 
plete, the  sinuses  occupy- 
ing the  central  parts  of 
the  node  with  the  spaces  at  the  periphery.  All  gradations,  in  fact,  are 
encountered,  from  the  typical  hemolymph  node  at  the  one  extreme  to  a 
lymph-node  with  enlarged  blood-vessels  on  the  other.  While  these  nodes 
share  in  the  production  of  lymphocytes,  they  are  probably  seats  of  destruction 
of  the  erythrocytes,  whose  remains  are  seen  in  the  phagocytes. 

Development  of  the  Lymphatic  System. — The  lymph-vessels 
probably  arise  from  the  veins  by  a  process  of  budding,  similar  to  that  fol- 
lowed in  the  extension  of  the  blood-vessels.  The  first  lymphatics  appear 
along  the  course  of  the  internal  jugular  vein,  as  a  series  of  outgrowths  from 
that  vessel.  These  spaces  fuse  to  form  a  lymph-channel  accompanying  the 
vein,  other  lymphatics  arising  in  a  similar  manner  in  connection  with  the 
subcardinal,  mesenteric  and  azygos  veins.  The  various  channels  thus 
formed  unite  to  form  a  continuous  system,  which  later  acquires  new  connec- 
tions with  the  subclavian  veins  near  their  junction  with  the  internal  jugulars. 


Fig.  152. — Modified   lymph-node,  containing  enlarged  blood-ves- 
sels and  approaching  type  of  hemolymph  node.     X  120. 


THE   SPLEEN. 


113 


The  origin  of  the  first  lymph-cells,  the  lymphocytes,  is  uncertain  (page  100), 
these  elements  appearing  outside  the  vessels  as  derivatives  of  the  mesoderm. 
After  the  establishment  of  the  lymphoid  tissue,  new  cells  are  continually 
being  formed  within  the  various  lymph-nodes  and  nodules.  The  lymph-nodes 
are  formed  by  two  fundamental  parts,  the  lymphoid  demerit,  consisting  of 
lymphocytes  in  a  reticulum  surrounding  the  terminal  artery  and  its  capilla- 
ries within  the  cords  and  germ-centres  respecti\'ely,  and  the  shms-elcment, 
represented  by  channels  resulting  from  multiplication  of  the  lymph-vessels. 
The  vascular  factor  is  constant  and  present  in  the  simplest  nodule;  the  sinus- 
element,  on  the  contrary,  varies,  sometimes  (as  in  the  usual  node)  being 
developed  from  closely  packed  lymph-ducts,  and  at  other  times  (as  in  the 
hemolymph  nodes)  being  venous  channels  occupied  by  blood.  By  the  sub- 
sequent intergrowth  of  the  lymphoid  element  and  the  greatly  multiplied 
lymph-capillaries,  the  intervening  bridges  of  connect''-e  tissue  are  reduced 
until  only  a  reticulum  remains,  the  lymphoid  tissue  being  brought  ultimately 
into  intimate  relation  with  the  surrounding  sinus. 

THE   SPLEEN. 

The  spleen  lies  far  back  on  the  left  side  in  the  abdominal  cavity,  between 
the  stomach  and  the  diaphragm,  and  measures  approximately  five  inches  in 
length  and  about  three  inches  in  width.      Its  form  is  variable  and  greatly  in- 


Capsule  — 


Primary  compartment. 


Interlobular  trabecula 


Intralobular  trabecula 


Interlobular  vein 


Splenic  nodule 


Capsule 


-^  -    —  X'etious  space 

—  Intralobular  vein 
Ampulla 
Arteriole 
Pulp-cord 
Venous  space 

Interlobular  trabecula 
containing  vein 


Splenic  artery 


Fig.  153.— Diagram  illustrating  architecture  of  a  splenic  unit ;   splenic  pulp  is  represented  in  only  one 

compartment.     {After  Mall.) 

fluenced  by  the  surrounding  organs,  since  its  substance  is  soft  and  yielding. 
It  contains  large  quantities  of  blood  and,  hence,  appears  of  a  dark  red  or 
purple  color.  The  spleen  may  be  classed  as  a  huge  hemolymph  node,  pos- 
sessing the  functions  of  producing  lymphocytes  and  destroying  erythrocytes. 
•The  spleen  is  enclosed  by  a  distinct  capsule,  which  consists  of  bundles 
of  dense  fibrous  tissue,  numerous  elastic  fibres,  and  sparsely  distributed  bundles 
of  unstriped  muscle.      With  the  exception  of  the  hilum,  the  area  between 


114 


NORMAL    HISTOLOGY. 


the  peritoneal  folds  at  which  the  splenic  vessels  and  nerves  enter  or  leave  the 
organ,  the  outer  surface  of  the  capsule  is  united  with  the  serous  membrane, 
the  peritoneum,  which  almost  completely  invests  the  spleen.  At  the  hilum 
the  tissue  of  the  capsule  is  continued  into  the  organ  and  supports  the  blood- 
vessels and  nerves.  The  capsule,  furthermore,  gives  off  from  its  deeper  sur- 
face numerous  processes,  the  trabecules,  which  pass  into  the  substance  of  the 
organ  and  break  up  into  innumerable  delicate  prolongations  that  unite  to 
form  the  supporting  fibrous  framework.  This  framework  is  arranged  with 
a  certain  degree  of  regularity,  since  the  trabeculae  subdivide  the  spleen,  at 
least  its  peripheral  zone,  into  fairly  regular  compartments,  the  splenic  lobules 
of  Mall,  about  i  mm.  in  diameter.  Each  of  these  units  is  imperfectly 
defined  by  three  interlobular  trabeculce,  from  which  secondary  intralobular 

Capsule- 


i^' 


6 


Interlobular 
'  trabecula 
and  vein 


o 
Interlob-  ■<f~ 
ularvein    ^ 


L.  Splenic 
h  pulp 


Splenic 
pulp 


Interlobular 
trabecula 


a^ 


Fig.  154. — Section  of  spleen  under  very  low  magnification,  showing  general  arrangement  of  splenic 

tissue.     X  10. 

processes  penetrate  the  lobule  and  subdivide  the  latter  into  about  ten  primary 
compartments.  These,  as  well  as  the  lobules  themselves,  are  not  isolated,  but 
freely  continuous,  since  the  intervening  trabeculae  and  processes  form  only 
incomplete  partitions.  The  spaces  within  the  fibrous  framework  are  filled 
with  the  highly  vascular  lymphoid  tissue,  known  as  the  splenic  pulp. 

The  relation  of  the  blood-vessels  to  the  splenic  lobules,  although  com- 
plex, is  very  definite.  The  branches  of  the  splenic  artery,  after  entering  at 
the  hilum  and  running  some  distance  within  the  trabeculae  in  company  with 
the  larger  veins,  break  up  into  smaller  vessels,  each  of  which,  parting  from 
the  vein,  enters  the  proximal  end  of  a  lobule,  through  the  middle  of  which  it 
courses,  giving  off  twigs,  one  for  each  primary  compartment  of  the  lobule. 
On  leaving  the  trabeculae,  the  arteries  carry  with  them  prolongations  of  con- 
nective tissue,  which,  with  the  adventiti^  of  the  vessels,  surround  the  latter 
with  fibro-elastic  coats  of  considerable  thickness.  Within  these  envelopes 
local  accumulations  of  lymphoid  cells  T  lymphocytes)  occur  and  in  conse- 
quence the  arteries  are  surrounded  by  spherical  or  fusiform  masses  of  dense 


THE   SPLEEN. 


115 


lymphoid  tissue,  the  splenic  nodules  or  Malpighian  bodies.  Depending  upon 
the  plane  of  section,  these  nodules  appear  in  preparations  of  the  spleen  as 
irregular  round  or  elongated  deeply  staining  tracts,  in  which  the  artery 
usually  lies  somewhat  eccentrically.  During  its  course  through  the  nodule, 
the  artery  gives  off  lateral  branches  which  are  resolved  into  capillaries  that 
pierce  as  well  as  supply  the  ensheathing  lymphoid  tissue. 

The  terminal  twigs  of  the  artery  are  the  small  short  vessels,  known  as 
the  pnlp-arterioles,  which  enter  the  anastomosing  strands  of  lymphoid  tissue, 
the  piilp-cords,  that,  together  with  the  blood-spaces,  constitute  the  splenic 
pulp  occupying  the  intervals  of  the  fibrous  framework.  After  repeated 
branching,  the  pulp-arterioles  give  rise  to  arterial  capillaries  possessed  of 
relatively  thick  walls.  The  further  course  of  the  blood-stream  probably 
varies,  since  some  of  the  arterial  capillaries  become  directly  continuous  with 
enlarged  thin-walled  channels,    the  ampidlcE   or  splenic  sinuses,   which   lie 


Germ-centre 


cord '''-^Fo  Vi>^lt«^^ 


Venous  space-  ~-"  '^^iSs;^,^       -  - i^'  '  ^7'^ jny^' 

FiG.  155. — Section  of  splenic  nodule,  showing  its  relations  to  surrounding  pulp-tissue.     X  120. 

between  the  pulp-cords  and  convey  the  blood  into  the  wide  venous  capillaries 
that  constitute  the  commencement  of  the  more  definite  intralobular  veins. 
Other  of  the  arterial  capillaries  and  the  capillaries  coming  from  the  splenic 
nodules  lose  their  walls,  the  blood  escaping  into  the  splenic  pulp,  which 
thus  becomes  infiltrated  with  great  numbers  of  erythrocytes  and  consequently 
appears  of  a  deep  red  tint.  After  slowly  welling  through  the  pulp,  during 
which  passage  the  efTete  erythrocytes  are  attacked  and  destroyed  by  the 
phagocytic  lymphoid  cells,  the  blood  passes  by  narrow  channels  into  the 
venous  spaces  and  radicles  forming  the  intralobular  veins.  It  is  probable, 
therefore,  that  while  one  part  of  the  blood  brought  to  the  spleen  finds  its 
way  actually  into  the  splenic  pulp,  another  part  may  pass,  by  a  closed  path 
and  under  usual  conditions,  from  the  arteries  into  the  veins,  without  ming- 
ling with  the  lymphoid  elements  of  the  splenic  pulp.  The  intralobular  veins 
are  tributary  to  the  larger  interlobular  veins,  which  occupy  the  interlobular 
trabeculae  and,  finally,  emerge  at  the  hilum  as  the  branches  of  the  splenic  vein. 
The  splenic  pulp  consists  of  an  intricate  complex  made  up  of  a  deli- 
cate supporting  reticulum,  continuous  with  the  terminal  ramifications  of  the 
intralobular  trabeculse,  and  the  cells  contained  within  and  supported  by  the 
meshwork,  together  with  the  thin-walled  splenic  sinuses  and  venous  chan- 
nels. The  pulp-cells  include  a  variety  of  elements,  the  most  constant  of 
which  are:   (a)  lymphocytes;    (b)  leucocytes  of  the  mononuclear  and  poly- 


ii6 


NORMAL    HISTOLOGY. 


morphonuclear  types;  (<:)  red  blood-cells;  {d)  phagocytic  cells  containing 
disintegrating  red-cells  or  pigment  particles  derived  from  the  destruction  of 
the  same;  and  {e)  large  multinucleated  cells.  A  variable  amount  of  free 
pigment  from  the  disintegrated  red  cells  is  also  present.  During  embryonic 
life,  and  perhaps  later  in  response  to  unusual  demands  for  new  red  blood- 
cells  (as  after  severe  hemorrhage),  the  spleen  is  the  birth-place  of  new 
red  cells;  these  are  at  first  nucleated,  but  soon  lose  their  nuclei.  The  ele- 
ments forming  the  imperfect  endothelial  lining  of  the  ampullae  or  splenic  si- 
nuses are  peculiar  in  being  elongated  and  possessed  of  nuclei  which  project 
into  the  lumina  of  the  channels.  They  are  sometimes  called  spleriic  fibres 
and  are  said  to  be  contractile.  The  reticular  tissue  is  disposed  around  the 
splenic  sinuses  and  venous  radicles  in  rings  which  probably  support  and 
prevent  collapse  of  the  delicate  channels.  The  splenic  nodules  correspond 
in   structure  with  the  cortical   nodules  of  lymph-nodes   and  often   enclose 


Arteriole 


Pulp-cords 


Venous  space 


Reticulum 


Arteriole 


Fig.  156. — Section  of  spleen,  showing  details  of  pulp-tissue.    X  300. 

germ-centres.  Since  within  the  spleen  the  worn-out  erythrocytes  are 
destroyed  and  new  lymphocytes  are  produced,  it  is  evident  that  the  blood 
carried  away  from  the  organ  by  the  splenic  vein  is  poorer  in  red  and  richer 
in  white  cells  than  that  brought  by  the  splenic  artery. 

The  lymphatics  are  represented  by  a  meagre  set  of  superficial  vessels, 
which  lie  beneath  the  serous  membrane  and  converge  towards  the  hilum. 

The  nerves,  derived  from  the  sympathetic  solar  plexus,  include  many 
nonmeduUated  fibres.  For  the  most  part  they  are  sympathetic  fibres  des- 
tined for  the  unstriped  muscle  within  the  walls  of  the  blood-vessels  and 
within  the  trabeculae.  They  enter  at  the  hilum  and  accompany  the  branches 
of  the  splenic  artery.  Delicate  nonmeduUated  fibres  have  been  described 
within  the  splenic  pulp,  some  of  which  are  presumably  sensory  in  function. 

Accessory  spleens  are  common,  but  they  are  not  all  of  the  same 
significance.      Some  are  isolated  parts   of  the  spleen,  which   have  become 


THE   CAROTID    BODY. 


117 


constricted  anci  eventually  separated.  Others  are  seemingly  independent 
masses  of  splenic  tissue.  Not  a  few  have  no  splenic  nodules  and  are  inter- 
mediate between  the  spleen  and  the  lymph-nodes,  and,  probably,  are  to  be 
classed  as  hemolymph  nodes. 


As  a  matter  of  convenience,  mention  may  be  made  at  this  place  of  two 
organs — the  carotid  and  the  coccygeal  bodies — concerning  whose  functions 
little  or  nothing  is  known.  The  systematic  position  of  these  structures  is  at 
present  uncertain,  but,  from  its  histological  characteristics,  the  carotid 
body  is  probably  to  be  regarded  as  closely  related  to  or,  in  a  sense,  an 
appendage  of  the  system  of  sympathetic  nerves,  whilst  the  coccygeal  body 
may  be  included,  with  seeming  propriety,  with  the  organs  of  internal  secre- 
tion. Their  grouping  and  description  here,  therefore,  must  be  understood 
to  be  a  matter  of  convenience  and  expediency  and  not  an  attempt  to  define 
their  true  relations. 

THE  CAROTID  BODY. 

This  organ,  also  known  as  the  glomus  carotictim,  carotid  gland  and 
ganglion  intcrcaroticiim,  is  a  small  ovoid  body  measuring  usually  about 
5  mm.  in  length,  from  2.5-4  mm.  in  width  and  about   1.5  mm.  in  thickness. 


Capillaries 


Capsule 


Fig.  157. — Section  of  carotid  body  of  adult  man  ;  one  entire  lobule  is  shown.     X  170- 

It  may  attain  a  length  of  7  mm.  and  exists  on  both  sides.  Its  most  frequent 
position  is  on  the  median  and  deep  side  of  the  upper  end  of  the  common 
carotid  artery  in  close  relation  with  the  point  of  division  of  the  latter  vessel 
into  the  external  and  internal  carotids.  The  body  usually  lies  not  within  the 
bifurcation,  but  rather  on  the  inner  side  of  the  common  carotid,  so  that  its 
form  and  relations  are  best  displayed  by  dissection  from  within  outwards. 

The  body  is  surrounded  by  a  thin  fibrous  capsule,  from  which  delicate 
septa  penetrate  inwards  and  divide  the  organ  into  a  small  and  uncertain 
number  (5-15)  of  spherical  masses  or  lobules,  from  .2-5  mm.  in  diameter, 
which  consist  of  a  complex  of  blood-vessels,  nerve-fibres  and  peculiar  cells. 


ii8  NORMAL    HISTOLOGY. 

The  latter  are  irregularly  disposed  as  clumps  or  cell-balls  and  occupy  the 
interspaces  within  the  close  network  of  large  capillaries  which  ramify  among 
the  cells.  The  characteristic  elements  of  the  carotid  body  are  the  polygonal 
cells,  about  lo  jj.  in  diameter,  with  large  round  nuclei.  Their  protoplasm  is 
finely  granular  and  is  especially  prone  to  change,  being  best  preserved  in 
solutions  of  chromic  acid  salts.  When  so  treated,  they  take  on  the  peculiar 
yellow  color  entitling  them  to  be  classed  as  chromaffine  cells.  The  large 
number  of  nerve-fibres  within  the  carotid  body  is  remarkable.  They  are 
mostly  nonmeduUated  and  are  derived  chiefly  from  the  neighboring  sympa- 
thetic plexus  surrounding  the  carotid  artery.  After  entering  at  different 
places,  they  ramify  within  the  organ  in  all  directions,  the  finest  filaments 
being  lost  among  the  groups  of  cells.  The  penetrating  nerve-trunks  usually 
enclose  typical  ganglion-cells  and,  in  a  sense,  the  chromaffine  cells  likewise, 
since  the  nerve-fibres  surround  the  groups  of  these  elements. 

In  view  of  (i)  the  identity  of  its  elements  with  other  chromaffine  cells, 
which  are  now  recognized  as  closely  associated  Avith  the  sympathetic  system 
in  other  localities,  as  in  the  medulla  of  the  suprarenal  body,  (2)  its  extraor- 
dinary richness  in  nerve-fibres,  (3)  its  general  resemblance  to  a  sympathetic 
ganglion,  and  (4)  its  direct  development  from  embrj'onal  sympathetic  gan- 
glion-cells, Kohn  concludes  that,  since  the  carotid  body  is  neither  a  gland  nor 
a  typical  ganglion,  it  must  be  regarded  as  accessory  to  the  sympathetic 
system  and,  in  recognition  of  this  relation,  proposes  the  name  par'aganglion 
caroticum  for  the  organ.  Concerning  its  function  nothing  is  definitely 
known. 

The  blood-vessels  supplying  the  carotid  body  are  branches  which  pass 
directly  from  either  the  common  carotid  artery  or  its  terminal  branches. 

THE  COCCYGEAL  BODY. 

This  organ,  also  often  called  xki^  glomus  coccygeum,  coccygeal  gland,  or 
Luschka  s  gland,  is  a  small  reddish  yellow  ovoid  body  which  lies  embedded 
in  fatty  areolar  tissue  usually  immediately  in  front  of  the  tip  of  the  coccyx, 
but  sometimes  just  below.  The  dimensions  of  the  organ  are  small,  its 
transverse  and  greatest  diameter  being  from  2.5—3  rnm.  and  its  thickness 
less  than  2  mm.  It  sometimes  is  divided  into  two  or  even  more  tiny  lobes. 
The  body  thus  described  is,  however,  only  the  largest  of  a  series  of  nodules 
which  includes  a  variable  number  of  structures,  for  the  most  part  of  minute 
size,  irregularly  grouped  around  the  chief  mass  (Walker).  The  additional 
nodules  are  in  many  cases  connected  with  the  principal  body  by  means  of 
delicate  pedicles;  in  others  they  are  entirely  free,  but  in  all  instances  they 
are  grouped  around  the  middle  sacral  artery  or  its  branches. 

The  body,  as  seen  in  transverse  sections  (Fig.  158),  includes  an  irreg- 
ularly oval  field  of  connective  tissue,  fairly  well  defined  from  the  surrounding 
fatty  areolar  tissue,  in  which  are  enclosed  numerous  aggregations  of  epi- 
thelial cells  and,  sometimes,  a  thick-walled  artery.  The  proportion  of  cell- 
masses  to  the  connective-tissue  stroma  varies,  in  some  cases  the  cellular 
constituents  predominating,  but  commonly  the  fibrous  stroma  being  the  more 
bulky.  The  individual  cell-groups  are  uncertainly  circumscribed  by  a  slight 
condensation  of  the  surrounding  fibrous  stroma.  Each  aggregation  of  cells 
contains  a  central  blood-space,  limited  by  an  endothelial  wall  similar  to  that 
of  a  capillary.  Against  this  wall  the  epithelial  cells  lie  without  the  interven- 
tion of  connective  tissue;  likewise  the  cells  themselves  are  closely  packed  in 
direct  apposition  with  one  another  and  in  consequence  present  a  polygonal 


MUCOUS    MEMBRANES. 


119 


contour.  They  are  disposed  around  the  central  \essel  in  from  two  to  five 
layers,  the  individual  cells  being  indistinctly  outlined  and  composed  of  clear 
protoplasm  containing  a  relatively  large  and  deeply-staining  nucleus.  Con- 
cerning the  mooted  question  as  to  the  presence  of  chromafifine  cells  within 
the  coccygeal  body,  the  testimony  as  to  their  absence  seems  convincing. 
The  cells  at  no  period  exhibit  the  chrome-reaction,  and  haxe  no  histogenetic 


r 


I  - 


Connective  tissue 
stroma^ 


Capillaries 


^~^^^-»~***>* 


Capsule . 


(  ells 
Llood  vcbbels 


*r    y 


Fig.  158. — Section  of  coccygeal  body  of  adult  man.     X  220. 

relation  to  the  sympathetic  system.  On  the  other  hand,  the  epithelial  char- 
acter of  the  cells,  their  intimate  relation  to  the  blood-vessels,  and  the  absence 
of  excretory  ducts,  seem  to  justify  the  inclusion  of  the  coccygeal  body,  at 
least  provisionally,  among  the  organs  of  internal  secretion. 


MUCOUS  MEMBRANES  AND   GLANDS. 

The  apertures  of  the  digestive,  respiratory  and  genito-urinary  tracts  on 
the  surface  of  the  body  mark  localities  at  which  the  integument  becomes 
continuous  with  the  walls  of  cavities  and  passages  communicating  with  the 
exterior.  The  linings  of  such  spaces  and  tubes  constitute  mucous  vieni- 
braiics.  The  latter,  however,  not  only  form  the  free  surface  of  the  chief 
tracts,  but  are  continuous  with  the  ducts  and  tubes  leading  into  the  glands, 
which  are  secretory  appendages  developed  as  outgrowths  from  the  mucous 
membranes.  These  membranes  line  two  great  tracts,  the  gastro-pidmonary 
and  the  genito-iiriiiary. 

THE    MUCOUS    MEMBRANES. 

Every  mucous  membrane  comprises  two  distinct  parts  :  the  epitheliitm, 
which  forms  the  immediate  free  surface  and  protects  the  delicate  subjacent 
structures,  and  the  tunica  propria,  a  connective-tissue  stroma  which  gives 
place  and  support  to  the  terminal  branches  of  the  blood-vessels  and  nerves 
and  the  beginnings  of  the  lymph-channels.  A  stratum  of  siibmncons  tissue, 
ordinarily  loose  and  extensible,  usually  connects  the  mucous  membrane  with 
the  surrounding  structures. 


I20 


NORMAL   HISTOLOGY. 


The  epithelium  may  be  squamous  or  columnar,  simple  or  stratified. 
Its  character  is  determined  largely  by  the  conditions  to  which  it  is  subjected 
or  by  its  function.  Thus,  where  a  mucous  membrane  is  exposed  to  the 
mechanical  influences  of  foreign  bodies,  the  epithelium  is  commonly  strati- 


Epithelium 

Papilla  with 
capillaries 


Tunica  propria 


Epithelium 


Fig.  159. — Section  of  oral  mucous  membrane,  showing  epithelium  and  tunica  propria.     X  300. 

fied  squamous,  as  in  the  upper  part  of  the  digestive  tract.  Where  concerned 
in  facilitating  absorption,  as  in  the  intestinal  tube,  it  is  simple  columnar  in  type. 
In  localities  in  which  the  existence  of  a  current  favors  the  function  of  an 
organ,  either  as  a  means  of  freeing  the  surface  from  secretion  or  particles  of 
foreign  matter,  as  in  the  respiratory  tract,  or  of  propulsion  through  a  tube, 

as  in  the  oviduct,  the  epi- 
thelium is  of  the  ciliated 
columnar  variety.  Modi- 
fications of  the  epithelial 
cells,  owing  to  the  presence 
of  pigment  or  of  secretion, 
distinguish  certain  mucous 
membranes,  as  those  of  the 
olfactory  region  and  the 
large  intestine  respectively. 
The  tunica  propria 
consists  of  a  stroma  of  inter- 
lacing bundles  of  fibro- 
elastic  connective  tissue, 
supporting  spindle-shaped 
or  stellate  connective  tissue 
cells.  The  latter  commonly 
lie  against  the  walls  of  the 
interfascicular  lymph-spaces 
that  occur  between  the  bun- 
dles of  stroma-tissue.  In  many  localities  the  outer  surface  of  the  tunica 
propria  is  beset  with  elevations,  the  papillcs,  over  which  the  epithelium 
e'xtends.  When  slight,  such  irregularities  may  not  modify  the  free  surface 
of  the  mucous  membrane,  since  the  epithelium  completely  fills  the  depres- 
sions between  the  elevations.  When  more  pronounced,  the  papillae  or 
folds  of  connective  tissue  produce  conspicuous    modelling  of   the  surface, 


Fig.  160. — Section  of  mucous  membrane  of  oesophagus.     X  55- 


MUCOUS    MEMBRANES.  121 

as  seen  in  the  papillae  of  the  tongue  or  the  rugst  of  the  vagina.  The  papilhe 
contain  the  terminal  loops  of  the  blood-vessels  and  many  of  the  endings  of 
the  sensory  nerves.  Where  increase  of  surface  is  desirable,  the  mucous 
membrane  may  be  thrown  into  cylindrical  elevations,  or  viV/i,  as  conspicuously 
seen  in  the  small  intestine.  In  many  places,  particularly  in  the  digestive 
tract,  the  mucous  membrane  contains  more  or  less  definite  accumulations 
of  lymphoid  tissue,  of  varying  size  and  complexity,  as  exemplified  by 
the  lymph-nodules  within  the  vermiform  appendix  and  the  Peyer  patches 
within  the  ileum.  A  more  or  less  definite  line  separates  the  epithelium 
from  the  subjacent  tunica  propria.  This  demarcation  is  the  basement 
membrane  or  membrana  propria.  Often  the  basement  membrane  appears 
as  a  mere  line  beneath  the  epithelium  and  is  then,  probably,  due  to  the 
apposition  of  the  bases  of  the  epithelial  cells.  Where,  as  around  glandular 
tissue,  it  is  well  developed  and  appears  as  a  definite  homogeneous  membrane, 
it  is  a  product  of  the  tunica  propria.  Exceptionally  a  reticular  structure 
is  recognizable.  Sometimes  the  deepest  stratum  of  the  mucous  membrane, 
next  the  submucous  layer,  is  occupied  by  a  narrow  sheet  of  involuntary 
muscle,  the  muscidaris  mucosts.  While  not  everywhere  present,  it  is  well 
developed  in  the  intestinal  tract  and  in  places  consists  of  two  distinct  strata, 
a  circular  and  a  longitudinal.  The  muscularis  mucosce  belongs  to  the 
mucous  membrane  and  must  not  be  confounded  with  the  muscular  coat 
proper  which  is  often  a  conspicuous  additional  tunic. 

The  submucous  layer,  the  stratum  of  areolar  tissue  connecting  the 
mucous  membrane  with  the  underlying  structures,  varies  in  thickness  and 
density.  Usually  the  attachment  is  a  loose  one  and  readily  permits  changes 
in  position  and  tension  of  the  mucous  membrane;  the  latter,  under  such 
conditions,  is  often  thrown  into  temporary  folds  or  rugae,  as  in  the  oesoph- 
agus and  stomach.  In  other  places  the  folds  are  permanent  and  not  effaced 
by  distention  of  the  organ,  as  conspicuously  demonstrated  by  the  plications 
in  the  duodenum  in  which  the  submucous  tissue  forms  the  basis  of  the  band- 
like elevations. 

The  blood-vessels  supplying  mucous  membranes  reach  the  latter  by 
way  of  the  submucous  tissue,  in  which  the  larger  arterial  branches  divide 
into  twigs  that  pass  into  the  mucosa.  Within  the  deeper  parts  of  the  tunica 
propria  the  arterioles  break  up  into  capillaries  forming  subepithelial  and 
papillary  networks,  the  vascular  loops  being  limited  to  the  connective-tissue 
stroma.  The  veins  usually  follow  the  arteries  in  their  general  course. 
When  glands  are  present,  the  capillaries  surround  the  tubules  or  alveoli 
with  rich  networks,  in  close  relation  with  the  basement  membrane.  The 
lymphatics  within  the  mucous  membrane  are  often  represented  by  lymph- 
spaces  between  the  bundles  of  stroma-tissue.  Towards  the  deeper  parts  of 
the  mucosa,  however,  more  definite  paths  exist  as  thin-walled  channels 
which  converge  towards  the  submucous  tissue.  Within  the  latter  the 
lymphatics  form  networks,  provided  with  valves  and  beset  with  the  accom- 
panying dilatations. 

The  nerves  distributed  to  mucous  membranes  include  medullated  and 
nonmeduUated  fibres  derived  from  the  cranial  or  spinal  trunks  and  the 
sympathetic  ganglia.  The  pale  sympathetic  fibres  are  destined  for  the  invol- 
untary muscle  of  the  stroma  and  of  the  blood-vessels  and  for  the  glands. 
The  immediate  supply  of  the  involuntary  muscle-cell  is  always  the  efferent 
fibre  (axone)  from  the  sympathetic  neurone,  which  is  thus  the  last  link  in 
the  chain  conducting  the  motor  impulse.  The  position  of  the  sympathetic 
cells  varies,  in  some  cases  being  remote  and  in  others  close  to  the  muscle 


122 


NORMAL   HISTOLOGY. 


supplied.  When  near  the  mucous  membrane,  they  occupy  the  microscopic 
gangHa  within  the  submucous  layer.  Surfaces  highly  endowed  with  sensi- 
bility are  generously  provided  with  twigs  containing  medullated  fibres. 
As  the  latter  approach  their  destination,  they  lose  their  medullated  char- 
acter and,  as  naked  axis-cylinders,  form  subepithelial  plexuses  from  which 
fibrils  pass  into  the  papillae,  where  some  terminate  in  free  or  special  end- 
ings. Others  enter  the  epithelium  and  penetrate  between  the  cells  for 
a  variable  distance  to  end  free,  for  the  most  part,  in  minute  bulbous 
swellings. 


Non-vascular  epithelium 


Terminal  capillary  loops 


-Tunica  propria 


Larger  branches  within 
submucosa 


Fig.  i6i. — Section  of  injected  oral  mucous  membrane ;  the  terminal  capillary  loops  occupy  the  papillae  of 

the  tunica  propria.     X  60. 


THE   GLANDS. 

Glands  are  essentially  outgrowths  from  the  epithelium  of  mucous 
membranes,  the  epithelial  elements  becoming  modified  into  gland-cells 
which  assume  the  role  of  secretion-forming  organs,  whose  products  are 
discharged  on  the  free  surface  and  keep  the  latter  moist.  This  last  pur- 
pose, however,  is  incidental  in  the  case  of  many  important  glands,  as  the 
parotid,  pancreas  or  liver,  since  these  organs  supply  secretions  for  partic- 
ular ends. 

The  simplest  type  is  the  unicellular  gland  found  in  the  lower  forms;  in 
principle  this  is  represented  in  man  and  the  higher  animals  by  the  goblet- 
cells  which  occur  in  profusion  in  mucous  membranes  covered  with  columnar 
epithelium.  The  viscid  secretion,  or  imicus,  poured  out  by  the  goblet-cells 
serves  to  protect  and  lubricate  the  surface  of  the  mucous  membranes.  The 
term  "gland,"  however,  commonly  implies  a  more  complex  organ,  com- 
posed of  an  aggregation  of  secretion-producing  cells  enclosed  within  connec- 


THE   GLANDS.  123 

tive  tissue  and  provided  with  ducts  and  blood-vessels.  The  application  of 
the  term  to  lymph-nodes  is  undesirable,  since  these  structures  do  not  secrete. 
Further,  designating  such  organs  as  the  thyroid,  parathyroid  and  suprarenal 
bodies  as  ' '  ductless  glands ' '  seems  less  satisfactory,  from  the  view-point  of 
accurate  terminology,  than  grouping  them  as  ' '  organs  of  internal  secre- 
tion."  Only  those  organs  which  produce  secretions  that  are  carried  oft 
through  definite  openings  or  ducts  are  entitled  to  the  term  gland. 

Glands  are  classified  according  to  their  form  into  two  chief  groups,  the 
tubular  and  the  alveolar,  each  of  which  occurs  as  simple  or  compound.  In 
many  instances,  however,  no  sharp  distinction  between  these  conventional 


Fig.  162. — Diagram  illustrating  types  of  glands,  a-e,  tubular;  f-i,  alveolar  or  saccular,  a,  simple  ;  b, 
coiled  ;  c,  d.  increasingly  complex  compound  tubular;  e,  tubo-alveolar ;  f,  simple;  g^  h,  t,  progressively 
complex  compound  alveolar. 

groups  exists,  some  important  glands,  as  the  salivary,  being  in  fact  a 
blending  of  the  two  types;  such  glands  are,  therefore,  appropriately  called 
htbo-alveolar. 

In  the  least  complex  type,  the  simple  tubular,  the  gland  consists  of  a 
cylindrical  depression  lined  with  epithelium  continuous  with  and  covering 
the  adjacent  surface  of  the  mucous  membrane,  as  an  outgrowth  of  which  it 
is  developed.  In  such  simple  glands  the  two  fundamental  parts,  the  fundus 
and  the  duct,  are  seen  in  their  primary  type.  The  fundus  includes  the 
deeper  portion  of  the  gland,  in  which  the  epithelium  has  assumed  the  secre- 
tory function,  the  cells  becoming  larger  and  more  spherical.  The  distinction 
between  spongioplasm  and  the  intervening  substances  is  usually  marked  in 
consequence  of  the  particles  of  secretion  (metaplasmj  stored  up  within  the 
meshes  of  the  spongioplastic  reticulum;  hence  the  latter  is  often  strikingly 
displayed.  The  duct  connects  the  fundus  with  the  free  surface  and  carries 
of?  the  secretion  produced  by  the  gland-cells.  It  is  lined  with  cells  that  take 
no  part  in  secretion  and  retain  for  some  distance  the  character  of  the  adjoin- 


124 


NORMAL   HISTOLOGY. 


ing  surface  epithelium.  Dilation  of  the  fundus  of  the  primitive  type  produces 
the  simple  alveolar  or  saccular  gland;  division  of  the  fundus  and  part  of  the 
duct  gives  rise  to  the  compound  tubular  variety ;  repeated  subdivision  of  the 
duct,  with  moderate  expansion  of  the  associated  terminal  parts,  leads  to  the 
production  of  the  tubo-alveolar  type. 

Simple  tubular  glands  may  be  minute  cylindrical  depressions  of 
almost  uniform  diameter,  as  the  crypts  of  Lieberkiihn  in  the  intestine,  or 
they  may  be  somewhat  wavy  and  slightly  expanded  at  the  fundus,  as  seen  in 
the  gastric  glands  towards  the  cardiac  end  of  the  stomach.  When  torsion 
becomes  very  pronounced,  as  in  the  sweat-glands,  the  coiled  gland  results. 
Compound  tubular  glands  present  all  degrees  of  complexity,  from  a 
simple  bifurcation  of  the  fundus  and  adjoining  part  of  the  duct,  as  in  the 
pyloric  glands,    to   the  elaborate   duct-system   ending  in  tubular  terminal 

divisions,    conspicuously  exampled 
in  the  kidney; 

Tubo-alveolar  glands,  mod- 
ified compound  tubular,  constitute 
a  very  important  group  comprising 
many  of  the  chief  secretory  organs 
of  the  body,  as  the  salivary  glands. 
They  are  made  up  by  the  repetition 
of  similar  structural  units,  differences 
in  the  size  of  the  organs  depending 
upon  the  number  of  associated  units. 
Each  unit  corresponds  to  the  groups 
of  terminal  compartments,  ox  alveoli, 
connected  with  a  single  ultimate 
division  of  the  duct-system.  The 
alveoli  or  acini  contain  the  secreting 
cells  and  are  limited  externally  by 
a  basement  membrane,  often  well 
developed,  which  supports  the  glan- 
dular epithelium  and  separates  the 
latter  from  the  perialveolar  blood- 
vessels that  surround  the  alveolus. 
The  alveoli  belonging  to  the  same 
terminal  duct  are  held  together  by 
delicate  connective  tissue  and  constitute  a  pyramidal  mass  of  glandular  tissue., 
the  primary  lobule.  The  primary  lobules  are  assembled  into  larger  groups, 
the  secondary  lobules,  which  in  turn  are  united  by  the  interlobular  connective 
tissue  as  the  lobes  of  the  gland.  The  lobes  are  held  together  more  or  less 
firmly  by  the  interlobar  areolar  tissue  continuous  with  the  general  fibrous 
envelope  or  capsule,  which  invests  the  entire  organ  and  separates  it  from  the 
surrounding  structures. 

The  interlobar  tissue  and  its  interlobular  continuations  contain  the  blood- 
vessels, lymphatics  and  nerves  supplying  the  gland  and,  in  addition,  the  major 
part  of  the  system  of  duct-tubes.  In  the  larger  glands  the  ducts  constitute  an 
elaborate  system  of  passages  arranged  after  the  general  plan  shown  in  the 
accompanying  diagram  (Fig.  163).  Traced  from  the  alveoli,  the  duct- 
system  begins  as  a  narrow  canal,  the  intermediate  d2ict,  lined  with  low 
cuboidal  or  flattened  cells  directly  continuous  with  the  glandular  epithelium 
of  the  alveoli.  After  a  short  course  the  intermediate  tube  increases  in 
diameter  and  becomes  the  intralobular  duct,  which  is  often  conspicuous  on 


Opening  on 
mucous  membrane 


Main  duct 


Interlobar  duct 


Interlobular  duct 


Intralobular  duct 

Intermediate  duct 

Beginning  of  duct 
Terminal  alveolus 


Fig.    163. — Diagram    illustrating    relations   of    duct- 
system  in  glands  of  tubo-alveolar  type. 


THE   GLANDS. 


125 


account  of  its  tall  and  sometimes  striated  epithelium.  The  further  path  of 
the  excretory  passages  lies  within  the  connective  tissue  separating-  the 
divisions  of  the  glandular  substance  and  embraces  the  interlobular  and  the 
iiitcrlobar  duds.  The  latter  join  to  form  the  usually  single  main  exn-etory 
dud  which  opens  on  the  free  surface  of  the  mucous  membrane.  The  excre- 
tory duct  is  lined  for  some  distance  by  cells  resembling  those  covering  the 
adjoining  mucous  membrane;  where  these  are  stratified  squamous  in  type, 
this  character  is  retained  for  only  a  short  distance  within  the  duct,  gradually 
giving  place  to  the  simple,  sometimes  at  first  double,  layer  of  columnar  epi- 
thelium which  extends  as  far  as  the  intralobular  ducts.  The  walls  of  the 
larger  ducts  consist  of  a  fibrous  coat,  containing  much  elastic  tissue  and  lined 


\i'Sf 


Mucous  alveoli 


"^r 


¥\ 


A'/ 


'Serous  a  veoli 


Fig.  164. — Section  of  tongue,  showing  alveoli  of  serous  and  mucous  types  of  glands.     X  60. 

by  epithelium;  in  the  large  glands,  as  in  the  parotid,  liver,  pancreas,  or 
testicle,  the  walls  are  strengthened  externally  by  a  layer  of  unstriped  muscle. 

The  glandular  epithelium  lining  the  ah'eoli  rests  upon  the  basement 
membrane  and  usually  consists  of  a  single  layer  of  spherical  or  polygonal 
secreting  cells.  The  latter  do  not  completely  till  the  alveolus,  but  leave  an 
intercellular  cleft-like  lumen  into  which  the  product  of  the  cells  is  poured  and 
from  which  the  secretion  passes  into  the  beginning  of  the  duct.  Depending 
upon  the  peculiarities  of  the  cells  and  the  character  of  their  secretion,  glands 
are  divided  into  serous  and  vuicoiis.  It  should  be  noted,  however,  that  in 
many  glands  both  serous  and  mucous  cells  occur,  either  within  adjoining- 
primary  lobules,  or,  indeed,  within  the  same  alveolus. 

The  serous  glands  are  distinguished  by  cells  which  are  distinctly 
granular,  somewhat  pyramidal,  and  provided  with  nuclei  situated  near  the 
centre.      The  secretion  elaborated  by  such  glands  is  thin  and  watery.      The 


126 


NORMAL    HISTOLOGY. 


general  appearance  of  the  cells  depends  upon  the  number  and  size  of  the 
particles  or  droplets  of  secretion  stored  within  their  cytoplasm  and  changes 
markedly  with  the  variations  of  functional  activity  of  the  gland.  When  a 
serous  gland  is  at  rest,  its  cells  are  loaded  with  secretion  and  appear,  therefore, 
larger  and  coarsely  granular.  After  active  function,  on  the  other  hand,  the 
cells  are  exhausted  and  appear  smaller  and  almost  free  from  granules,  often 
exhibiting  a  differentiation  into  a  clear  outer  zone,  devoid  of  granules,  and  a 
darker  inner  zone,  next  the  lumen,  in  which  secretion-granules  still  remain. 
The  mucous  glands  elaborate  a  clear  viscid  homogeneous  secretion, 
which  when  present  in  quantity,  as  during  rest,  distends  the  cells,  crowding 
the  nuclei    against  the    basement  membrane  and   giving  the  cells   a    clear 

and  transparent  character. 
When  loaded  and  distended 
with  secretion,  the  transpar- 
ent cells  have  well-defined 
outlines  and  a  narrow 
peripheral  zone  containing 
the  displaced  nucleus  and 
granular  cytoplasm.  After 
prolonged  activity,  the  ex- 
hausted cells  contain  rela- 
tively little  secretion,  hence 
the  threads  of  spongioplasm 
are  no  longer  separated 
but  near  together.  In  con- 
sequence, the  cells  lose  their 
transparency  and  become 
smaller,  darker  and  more 
granular  than  when-  the 
gland  is  resting. 

The  alveoli  of  mixed 
mucous  glands  often  contain 
crescentic  groups  of  small 
cells  lying  between  the  usual 
large  clear  elements  and  the 
basement  membrane  (Fig. 
165).  These  are  the  demi- 
lunes. Although  opinions 
differ  as  to  their  nature,  it  is 
probable  that  ordinarily  they 
are  aggregations  of  serous  cells.  In  order  to  afford  means  of  escape  for  their 
secretion,  since  the  serous  cells  are  excluded  from  the  lumen  of  the  alveolus  by 
the  mucous  elements,  minute  intercellular  channels,  the  secretion-canaliculi, 
pass  from  the  main  lumen  to  the  demilunes  (Fig.  166).  Such  secretion- 
canaliculi  are  not  limited  to  mixed  mucous  glands,  but  are  found  in  serous 
alveoli  and  in  other  glands  containing  isolated  secreting  cells,  as  in  the  peptic 
glands  of  the  stomach  (Fig.  198).  Probably  not  all  demilunes  are  composed 
of  serous  cells,  since  small  groups  of  mucous  cells,  when  containing  little 
secretion,  become  peripherally  displaced  by  the  distended  cells  and  then 
appear  as  crescents.  These,  however,  are  not  provided  with  secretion- 
canaliculi.  Minute  secretion-channels  have  been  described  within  the  cyto- 
plasm of  glandular  epithelium,  as  the  liver  cells;  it  is  questionable,  however, 
whether  such  intracelhdar  secretion-canaliculi  are  not  artificial  products. 


Demilune  of 
serous  cells 


Duct 


Mucous  cells 


Demilune 


Fig.  165. — Section  of  sublingual  gland,  showing  serous  cells 
grouped  as  demilunes.     X  245. 


THE   GLANDS. 


127 


Fig.  166. — Portions  of  salivary  glands,  showing  terminal 
ducts  and  secretion-canaliculi  ;  A,  from  submaxillary  of 
dog — the  canaliculi  extend  to  the  demilunes  of  serous 
cells;  B,  from  submaxillary  of  rabbit — the  canaliculi  pass 
between  the  serous  cells.     X  500 and  290.     {Retzius.) 


According  to  the  proportions  of  the  two  types  of  alveoH,  the  tubo-alveolar 
glands  have  been  arranged  in  four  groups:  {a)  pure  serous  glands,  in  which 
only  serous  alveoli  are  present,  as  the  parotid;  i^b)  mixed  serous  glands,  in 
which  a  few  mucous  alveoli  are 
intermingled  with  the  serous,  as 
in  the  submaxillary;  (r)  mixed 
miicous  glands,  in  which  the 
serous  cells  occur  as  demilunes, 
as  in  the  sublingual  and  buccal  ; 
and  (yd)  pure  mucous  glands,  in 
which  only  mucous  alveoli,  with- 
out demilunes,  are  found,  as  in 
the  palatal. 

Simple  alveolar  glands 
in  their  typical  flask-like  form, 
abundant   in    the    skin    of    the 
lower    vertebrates,    are    repre- 
sented by  the  simple  sebaceous 
glands.     The    dilated    sac-like 
fundus      is    lined     with     clear 
and  distended  cells,   which    become    modified    into    duct-cells  at    the  exit. 
Compound  alveolar  glands  consist  of  a  number  of  saccular  alveoli 
that  open  into  a  common  duct,  as  in  the  case  of  the  large  sebaceous  glands 
and    the  tarsal  glands  of  the  eyelid;  or  they  maybe 
much  more  complex,   being  made  up  of  a  number 
of  alveolar  systems,  the  ducts  of  which  join  a  large 
excretory  passage.      When  of  such  composition  they 
strongly  resemble  the  tubo-alveolar  type,  the  saccular 
character  of  the  alveoli  being  the  chief  distinction. 
The  parotid  and  the  serous  part  of  the  submaxillary 
are   regarded    by  some    histologists  as  examples  of 
compound  alveolar  glands.      The  lung  affords  a  con- 
spicuous example  of  the  principle  of  the  compound 
saccular  type  in  its  mode  of  development  and  the 
arrangement  of  the  air-tubes  and  saccular  terminal 
compartments. 

The  blood-vessels  distributed  to  glands  are 
always  numerous,  since  an  adequate  blood-supply  is 
necessary  to  bring  to  the  gland-cells  the  materials 
from  which  their  cytoplasm  may  select  the  substances 
required  for  their  metabolism  and  secretory  activity. 
In  the  case  of  the  smaller  and  simpler  glands,  the 
capillaries  of  the  mucous  membrane  form  a  meshwork 
outside  the  basement  membrane  enclosing  the  glan- 
dular epithelium.  The  large  compound  glands  are 
pro\'ided  with  vessels  whose  general  arrangement 
corresponds  with  that  of  the  duct-system,  the  blood- 
vessels following  the  tracts  of  interlobular  connective 
tissue  and  its  extensions  between  the  alveoli.  On 
reaching  the  latter  the  capillaries  form  networks  that 
overlie  the  basement  membrane  and  thus  bring  the  blood-current  into  close, 
but  not  direct,  relation  with  the  secreting  cells.  When  the  relation  between 
the  capillaries  and  the  cells  is  unusually  intimate,  as  it  is  in  the  liver  or  the 


>^. 


Fig.  167. — Injected  gastric 
mucous  membrane,  showing 
capillary  network  surround- 
ing tubular  glands.      <  50. 


128 


NORMAL    HISTOLOGY. 


Fig.  i68. — Portion  of  submaxillary  gland 
of  rabbit,  showing  distribution  of  nerves 
to  the  alveoli.     X  290.     (Retzius.) 


cortex  of  the  suprarenal  body,  a  basement  membrane  is  wanting,  a  delicate 
reticulum  and  the  wall  of  the  vessel  alone  intervening  between  the  blood- 
stream and  the  protoplasm  of  the  cells.  Although  subject  to  local  deviations, 
as  in  the  liver,  the  veins  follow  the  general  course  of  the  arteries,  the  larger 

blood-vessels,  together  with  the  duct-tubes, 
the  lymphatics  and  the  nerves,  occupying 
the  tracts  of  connective  tissue  between  the 
lobes  and  the  lobules.  The  lymphatics 
are  represented  by  the  trunks  which  accom- 
pany the  ducts  within  the  interlobular 
tissue.  Within  the  lobules  the  lymph- 
channels  become  less  definite  until,  finally, 
they  are  recognizable  only  as  the  lymph- 
spaces  between  the  bundles  of  connective 
tissue  separating  the  alveoli. 

The  nerves  distributed  to  the  larger 
glands  include  both  medullated  and  non- 
medullated  fibres  which  follow  the  arteries 
and  ducts,  around  which  they  form  plexuses. 
Along  these  strands  sympathetic  ganglion- 
cells  occur,  sometimes  singly  but  more  often 
grouped  as  microscopic  ganglia,  from  which 
sympathetic  fibres  proceed  to  the  muscle  of  the  blood-vessels  and  ducts  and 
to  the  alveoli.  Upon  reaching  the  latter,  the  nonmeduUated  fibres  break  up 
into  end-plexuses  surrounding  the  alveoli;  the  ultimate  distribution  includes 
epilemmar  and  hypolemniar  fibrillcs^  the  former  lying  upon  and  the  latter 
beneath  the  basement  membrane.  The  hypolemmar  fibrillae,  derived  from 
the  extra-alveolar  plexus,  pass  through  the  basement  membrane  and  end  in 
fine  varicose  threads  between  the  gland-cells. 

Development. — Since  glands  are  only  ex- 
tensions of  the  mucous  membrane  or  integument 
upon  which  they  open,  their  development  begins 
as  an  outgrowth  or  budding  from  the  epithelium 
covering  such  surfaces.  In  the  simple  tubular 
glands  the  minute  cylinders  are  closely  placed  and 
composed  of  densely  packed  cells.  In  the  case 
of  the  larger  compound  glands,  as  the  salivary 
or  pancreas,  the  first  anlage  consists  of  a  solid 
cylindrical  plug  which,  penetrating  into  the  meso- 
derm, soon  begins  to  branch.  The  ends  of  the 
terminal  divisions  enlarge  and  eventually  become 
the  alveoli.  Meanwhile  the  surrounding  meso- 
blast  undergoes  condensation  and  forms  the 
interlobular  and  other  septa,  as  well  as  the  general 
envelope,  or  capsule,  thereby  giving  definite  form 
to  the  glandular  aggregation.  The  vascular  and 
other  structures  usually  found  within  the  inter- 
lobular tissue  are  secondary  and  later  formations. 
The  development  of  the  gland  involves  a  double  process  of  active  growth, 
the  extension  of  the  epithelial  processes  and  a  coincident  subdivision  of  the 
latter  by  the  mesoderm  to  form  the  units  of  the  organ.  The  lumen  of  the 
gland  appears  first  in  the  main  excretory  duct,  from  which  it  extends  into 
the  secondary  tubes  and,  finally,  into  the  alveoli; 


IL  Alveoli, 
still  solid 


Fig.  i6g. — Section  of  foetal  oral 
mucous  membrane,  showing  devel- 
oping tubo-alveolar  gland.     X  50. 


THE  ALIMENTARY  CANAL. 

This  long  and  complicated  tube,  extending  from  the  mouth  to  the  anus, 
is  developed  from  the  entoderm  with  a  mesodermic  envelope,  except  at  the 
two  ends,  each  of  which  is  at  first  a  pouch  lined  by  ectoderm.  It  consists 
of  the  mouth,  pharynx  and  oesophagus  above  the  diaphragm,  and  of  the 
stomach  and  small  and  large  intestine  below  the  diaphragm.  There  are 
many  accessory  organs  connected  with  the  alimentary  canal  whose  primary 
function  is  to  assist  in  the  processes  of  digestion.  The  most  important  of 
these  above  the  diaphragm  are  the  teeth,  the  tongue  and  the  salivary  glands; 
those  below  the  diaphragm  are  glands  of  various  kinds,  mostly  so  small  as 
to  be  contained  within  the  mucous  membrane.  Two  large  organs,  however, 
the  li\'er  and  the  pancreas,  belong  to  this  class,  both  being  primarily  out- 
growths from  the  early  gut-tube.  The  general  structural  plan  of  the  alimen- 
tary canal,  presenting  in  places,  however,  great  modifications,  includes:  (i) 
a  lining  of  jniicous  membrane ;  (2)  a  submucous  layer  of  connective  tissue 
into  which  glands  may  penetrate  from  the  mucosa;  (3)  a  double  layer  of  un- 
striped  imiscle,  arranged,  for  the  most  part,  as  an  inner  circular  and  an 
outer  longitudinal  stratum;  and,  below  the  diaphragm,  (4)  2i  serous  covering 
from  the  peritoneum,  which,  although  originally  complete,  is  in  the  adult 
wanting  in  certain  parts. 

THE   ORAL   CAVITY. 

The  Mucous  Membrane. — The  histological  transition  from  the  skin 
covering  the  exterior  of  the  lips  to  the  oral  mucous  membrane  takes  place 
gradually,  the  two  being  connected  by  a  broad  intermediate  zone  which 
approximately  corresponds  to  the  red  area  of  the  hps.  The  oral  mucous 
membrane  is  everywhere  covered  with  stratified  squamous  epithelium,  from 
.2-.  4  mm.  in  thickness,  which  presents  the  details  of  the  varying  strata  of 
cells  typical  of  such  structures  (page  17).  When  for  any  reason  the  large 
flat  surface  cells  are  not  removed,  as  ordinarily  they  continually  are  by  abra- 
sion, they  form  a  whitish  semiopaque  film  that  masks  the  rosy  tint  of  the 
oral  mucosa.  The  haiica  propria  consists  of  closely  felted  bundles  of  fibrous 
tissue  and  elastic  fibres,  and  passes  into  the  submucous  stratum  without 
sharp  demarcation.  Towards  the  surface  supporting  the  epithelium,  the 
bundles  become  more  delicate  and  closer,  so  that  the  stroma  acquires  a  less 
fibrous  and  more  homogeneous  appearance.  The  subepithelial  border  of  the 
tunica  propria  is  beset  with  innumerable  minute  elevations,  the  papillcB, 
which  are  especially  well  developed  on  the  lips,  the  anterior  part  of  the  hard 
palate  and  the  gums.  The  papillae  of  the  tongue  are  special  structures  and 
are,  therefore,  here  not  considered.  Within  the  elevations,  which  contain 
the  vascular  loops  and  nerv^es,  the  stroma  is  relatively  compact  and  homo- 
geneous. In  addition  to  the  ordinary  connective  tissue  cells,  leucocytes  and 
mast-cells  are  frequently  encountered  within  the  stroma;  the  mast-cells,  dis- 
tinguished by  their  coarse  basophilic  granules,  are  particularly  abundant  in 
the  gum,  close  to  the  neck  of  the  tooth.  The  oral  mucous  membrane  is 
attached  to  the  surrounding  bones  and  muscles  by  the  submucous  layer,  a 
stratum  of  generally  loose  fibro-elastic  tissue  containing  the  larger  blood- 
vessels, lym{)hatics  and  nerve-trunks  and  the  small  oral  glands.  According 
9  129 


I30 


NORMAL   HISTOLOGY. 


to  the  amount  and  character  of  this  layer  the  mobiHty  of  the  oral  lining  varies. 
Where  plentiful  and  loose,  as  over  the  floor  of  the  mouth,  the  mucosa  is 
freely  movable,  while  over  the  hard  palate  and  the  alveolar  processes  of  the 
jaws  the  submucous  tissue  is  so  meagre  that  the  mucous  membrane  is  almost 
directly  blended  with  the  periosteum  and  correspondingly  fixed. 

The  blood-vessels  supplying  the  oral  mucous  membrane  are  numer- 
ous, the  larger  stems  occupying  the  submucous  layer,  within  which  they 
form  a  wide-meshed  network.      Thence    the   twigs    pass   into    the  tunica 


Labial  glands 


Fibres  of 
orbicularis  oris 


Transition  into 

true  mucous 

membrane 


Modified  mucous 
membrane 


Integument 


Sebaceous  gland 


Transition  into 
odified  skin 


Fig.  170.— Sagittal  section  of  lip  of  young  child,  showing  transition  of  skin  into  oral  mucous 

membrane.     ,  ,  20. 

propria,  \vhere  they  form  a  second  and  closer  network.  Their  ultimate  dis- 
tribution includes  capillary  loops  that  occupy  the  papillae,  the  smaller  eleva- 
tions containing  only  one  or  two  terminal  loops  and  the  large  ones  a  tuft  of 
half  a  dozen  or  more.  The  lymphatics  are  represented  by  a  network  of 
lymph-spaces  within  the  tunica  propria  which  drain  into  the  wide-meshed 
reticulum  of  definite  lymph-channels  within  the  submucous  layer.  The 
nerves  are  chiefly  meduUated  fibres  that  assume  a  loose  plexiform  arrange- 
ment within  the  submucosa.  From  here  numerous  twigs  enter  the  tunica 
propria  and  break  up  into  the  component  fibres,  which  mostly  terminate  in 
the  stroma  and  papillae  either  free  or  in  connection  with  end-bulbs  or  tactile 
corpuscles.  A  few  fibres,  after  losing  the  medullary  coat,  penetrate  the 
epitheliurn  and,  after  repeated  branching,  end  between  the  epithelial  cells  as 
naked  axis-cylinders. 


THE    ENAMEL. 


131 


Stripes  of  Retztus 
(longitudinal) 


Neck 


Prism-stripes  of 
Schreger  (I 
and  dark) 


^'1 

'■^ —  Schreger  (light 


THE   TEETH. 

In  principle  the  teeth  may  be  regarded  as  hardened  papillae  of  the  oral 
mucous  membrane;  they  consist,  therefore,  of  two  fundamental  parts,  the 
connective  tissue  body  or  core  and  the  epithelial  capping.  The  primary 
tissues  become  greatly  modi- 
tied  and  give  rise  to  the  three 
constituents  of  the  typical 
mammalian  tooth,  of  which 
the  enamel  is  deri\-ed  from 
the  ectodermic  epithelium, 
and  the  dentine,  with  the  pulp, 
and  the  eeniention  are  pro- 
duced bv  the  mesoderm. 

The  Enamel. —This, 
the  hardest  tissue  of  the  body, 
co\"ers  the  crown,  the  part  of 
the  tooth  projecting  beyond 
the  gum,  and  is  thickest  on 
the  cutting  edge  or  grinding 
surface.  It  gradually  thins 
ofT  towards  the  neck,  around 
which  its  terminal  border  ap- 
pears as  a  wavy  or  serrated 
line.  The  remarkable  hard- 
ness of  the  enamel  is  due  to 
the  excessive  amount  (97  per 
cent. )  of  earthy  material  and 
the  small  proportion  (3  per 
cent.  )  of  organic  matter  which 
it  contains.  The  enamel — 
the  product  of  epithelial  cells, 
the  amelohlasts — consists  of 
an  aggregation  of  h\'e-  or 
six-sided  columnar  elements, 
the  enamel-prisms,  which 
measure  from  3-5  mm.  in 
length  and  from  3-5  a  in 
width.  Their  general  dispo- 
sition ib  at  right  angles  to 
both  the  surface  of  the  den- 
tine, upon  which  they  rest, 
and  the  exterior  surface  of 
the  crown.  Since  the  prisms 
usually  extend  the  entire 
thickness  of  the  enamel,  they  are  of  slightly  larger  diameter  at  the  surface 
of  the  tooth  than  next  the  dentine.  Thev  run  for  a  short  distance  almost 
at  right  angles  to  the  surface  of  the  dentine,  then  bend  laterally  for  a  con- 
siderable part  of  their  course,  but  reassume  a  vertical  path  on  approaching 
the  external  surface.  In  addition  to  these  general  curves,  the  ranges  of 
enamel-prisms  have  a  spudl  arrangement,  in  consequence  of  which  the 
parallelism  of  the  prisms  is  disturbed  and   the  bundles  appear  in  sections 


R 
^>^ 


—Gum 


-Pulp-tissue 


Dentine 


i_Jj, —Cetnsiitum 


^      periosteum 


'^l 


Osseous 
'tissue  of  jaw 


%1 


'W 


Root  (.anal 


sC"  i.>t?-' 


FlG.  171. — Sagittal  section  of  canine  tooth  in  situ. 
grammatic. 


Semidia- 


132  NORMAL   HISTOLOGY. 

as  interwoven.  In  thin,  accurately  transverse  sections,  enamel  presents 
a  mosaic  of  minute  hexagons,  which  are  the  ends  of  the  cut  individual 
prisms.  Each  prism  consists  of  a  darker  central  and  a  lighter  peripheral 
zone,  which  depend  upon  variations  of  density.  The  lighter  peripheral 
zone  probably  represents  a  film  of  less  completely  calcified  substance  and 
is  often  interpreted  as  cement  material  holding  the  prisms  together.  After 
decalcification  and  staining,  the  true  cement  substance  may  be  distinguished 
as  delicate  lines  defining  the  prisms.  Particularly,  but  not  necessarily, 
after  the  action  of  acids,  the  enamel-prisms  in  longitudinal  sections  exhibit 
alternate  light  and  dark  transverse  markings  with  seemingly  beaded  or 
varicose  outlines.  These  appearances  are  probably  optical  and  depend 
upon  the  wavy  contour  of  the  central  denser  substance  of  the  prisms. 
The  true  outlines  of  the  prisms  are  straight,  the  opposed  surfaces  of 
the  adjoining  columns  being  separated  by  an  uniform  thin  layer  of  the 
cement-substance. 

When  an  axial  longitudinal  section  of  a  ground  tooth  is  examined  by 
reflected,  not  transmitted,   light,  the  enamel  exhibits  a  series  of  alternate 
dark  and   light  bands,    known   as   the  prism-stripes  of  Schreger.     These 
markings  (Fig.  171),  which  are  comparatively  coarse 
■"■'*"•'  and  generally  vertical  to  the   surface    of   the  tooth, 

depend  upon  the  relation  of  the  ranges  of  enamel- 
prisms  to  the  axes  of  the  rays  of  light.  Rotation  of 
the  illuminating  pencil  through  180°  changes  the  dark 
stripes  to  light  ones  and  vice  versa.  Each  stripe 
includes  from  ten  to  twenty  enamel-prisms  and  is 
invisible  by  transmitted  light.  In  addition  to  the 
foregoing  markings,  the  enamel  often  presents,  in 
radial  longitudinal  sections,  brownish  parallel  lines, 
the  stripes  of  Retzius,  which  correspond  in  their 
general  direction  with  the  contour  of  the  tooth,  but 
run  at  an  angle  of  from  15°  to  30°  with  the  free 
surface.  In  cross  sections,  these  stripes  are  repre- 
sented by  a  series  of  concentric  lines  encircling  the 
Fig.  172.— Ground  section    crown,  parallel  to  and  near  the  surface;  in  the  middle 

of  enamel,  showing  ranges  of  j     j  ^         r    ^-u  1^1.  u    1 

enamel-prisms.   X  500.  and  deeper  parts  of  the  enamel  they  are  much  less 

evident  or  entirely  wanting.  The  significance  of  the 
stripes  of  Retzius  is  still  disputed,  but  it  is  probable,  since  they  surely  do 
not  depend  upon  pigment,  that  they  are  due  to  local  imperfections  of  the 
calcification  of  the  enamel-prisms  during  certain  periods  of  the  growth  of 
the  enamel. 

The  enamel-adicle ^  or  membrane  of  Nasviyth,  is  a  delicate  envelope  that 
completely  invests  the  crown  of  the  newly-erupted  tooth.  In  the  course  of 
time  it  disappears  from  the  areas  exposed  to  wear,  but  over  the  protected 
surfaces  it  may  persist  throughout  life.  The  membrane  (from  2—4  i-i.  in 
thickness)  is  transparent,  structureless  and  resistent  to  the  action  of  acids, 
less  so  to  alkalies,  and  affords  protection  to  the  subjacent  enamel.  Since 
the  cuticle  is  not  only  continuous  with  the  cortical  substance  of  the  enamel- 
prisms,  but  also  agrees  with  it  in  optical  and  chemical  properties,  the  origin 
of  the  membrane  may  be  referred  to  the  enamel-producing  elements,  the 
epithelial  cells  forming  the  inner  layer  of  the  enamel-organ  Tpage  140). 
After  the  completion  of  their  work  in  producing  the  enamel-prisms,  they 
produce  a  continuous  envelope,  which  never  undergoes  calcification  and 
remains  as  the  enamel-cuticle. 


THE    DENTINE. 


133 


The  Dentine. — The  dentine  or  ivory,  the  substance  which  contributes 
the  bulk  of  the  tooth,  encloses  the  cavity  containing  the  pulp  and  is  itself 
surrounded  by  the  enamel  and  the  cementum.  In  both  its  genesis  and 
chemical  composition,  dentine  resembles  bone,  like  the  latter  being  a  con- 
nective tissue  modified  by  impregnation  with  lime-salts.  Dentine  exceeds 
bone  in  hardness  and  contains  a  larger  proportion  (72  per  cent.}  of  earthy 
matter  and  a  smaller  amount  (28  per  cent.)  of  organic  substance.  After 
decalcification  with  acids,  the  remaining  animal  material  retains  the  previous 
form  of  the  dentine  and  yields  gelatin  on  prolonged  boiling  in  water.  Den- 
tine, like  bone,  is  formed  through  the  agency  of  specialized  connective  tissue 
cells,  the  odontoblasts,  but 
differs  from  it  in  the  small 
number  of  these  cells  which 
become  imprisoned  in  the 
intercellular  matrix.  When 
this  occurs,  as  it  occasional- 
ly does,  the  dcntinc-ce/ls  cor- 
respond  to  the  bone-cells, 
both  being  connective  tis- 
sue elements  within  lymph - 
spaces  in  the  calcified  inter- 
cellular substance. 

Examined  in  dried  sec- 
tions under  low  magnifica- 
tion, the  dentine  exhibits  a 
radial  striation,  composed 
of  fine  dark  lines  which 
extend  from  the  pulp-cavity 
internally  to  the  enamel  ex- 
ternally. These  dark  lines 
are  the  dentinal  tubules, 
filled  with  air,  which  are 
homologous  with  the  lacunae 
and  canaliculi  of  bone  and 
contain  the  dentinal  fibres, 
as  the  processes  of  the  odon- 
toblasts are  called.  In  the 
crown,  as  seen  in  longitudi- 
nal sections,  the  course  of 
the  dentinal  tubules  is  radial 

to  the  pulp-cavity;  in  the  body  and  fang  their  course  is  approximately 
horizontal  and  almost  parallel.  The  canals,  however,  are  not  straight  but 
wavy,  the  first  bend  being  directed  towards  the  root  and  the  second  towards 
the  crown.  In  addition  to  these  primarv  curves,  especially  marked  in  the 
crown,  the  tubules  exhibit  numerous  shorter  secondary  curves,  the  whole 
arrangement  imparting  to  the  individual  canals  a  spiral  course.  In  conse- 
quence of  the  uniformity  of  these  curvatures,  the  tooth-ivory  displays  a 
series  of  linear  markings,  Schreger'  s  lines,  which  parallel  the  outer  surface 
of  the  dentine.  These  markings,  however,  must  not  be  confounded  with 
another  set  of  striae,  the  contour  lines  of  Ozceji,  or  the  increnie^ital  lines  of 
Salter,  which,  best  seen  in  the  crown,  run  obliquely  to  the  surface  of  the 
dentine  (Fig.  171)  and  depend  probably  upon  variations  in  calcification 
incident  to  the  growth  of  the  dentine. 


Fig.  173.— Ground  section  of  human  tooth   including  adjoining 
enamel  and  dentine.     X  280. 


134 


NORMAL    HISTOLOGY. 


The  dentinal  tubules  are  minute  canals  (from  1—2  //  in  diameter), 
which  begin  at  the  pulp-cavity,  where  they  are  largest,  and  extend  to  the 
outer  surface  of  the  dentine,  to  end  beneath  the  enamel  or  the  cementum. 
Each  spirally  coursing  canal  undergoes  branching  of  two  kinds,  a  dichoto- 
mous  division  at  an  acute  angle  in  the  vicinity  of  the  pulp-cavity,  resulting 
in  two  canaliculi  of  equal  diameter,  and  a  lateral  branching  during  the  outer 
third  of  their  course,  whereby  numerous  tubuli  of  diminishing  size  are  given 
off.  The  dentinal  tubules  are  occupied  by  the  de7itinal  fibres,  the  processes 
of  the  odontoblasts  within  the  pulp,  which  in  the  young  tooth  are  proto- 


Pulp-tissue  -cj 


Granular  layer  of    ^ 
dentine 


Cementum 


Alveolar  periosteum 


Fig.  174- — Transverse  section  of  root  of  lower  canine  tooth.     X  30. 

plasmic  threads;  later  they  lose  this  character  and  become  harder  and  stiffen 
The  dentinal  tubules  differ  in  their  mode  of  ending  in  the  crown  and  the 
root.  In  the  former  situation,  the  outer  surface  of  the  dentine  is  indented 
with  small  crescentic  depressions,  filled  with  enamel,  in  which  the  tubules 
abruptly  end,  as  if  cut  off.  On  the  root,  where  the  surface  of  the  dentine  is 
sm.ooth  and  covered  with  cementum,  the  tubules  end  in  curves  or  loops 
beneath  the  cementum,  only  in  exceptional  cases  communicating  with  the 
canaliculi  of  the  cementum.  The  immediate  walls  of  the  dentinal  tubules 
are  formed  by  delicate  membranes,  the  dentijial  sheaths  of  Neumann,  which 
are  specialized  parts  of  the  intertubular  matrix  of  greater  density  and  less 
complete  calcification.  After  softening  the  decalcified  dentine  by  alkalies, 
the  sheaths  may  be  isolated,  since  they  resist  the  action  of  reagents  which 
attack  the  surrounding:  substance. 


THE   CEMENTUM. 


135 


Dentine 


Granule  layer 


Tlie  intertubular  dentine-matrix  resembles  that  of  bone  in  being  com- 
posed of  bundles  of  extremely  delicate  fibrous  fibrillae  that  swell  on  treatment 
with  water  containing  acids  or  alkalies  and  yield  gelatin  after  prolonged 
boiling  in  water.  The  disposition  of  the  bundles  of  fibrillae,  more  regular  in 
dentine  than  in  bone,  is  chiefly  longitudinal  and  parallel  to  the  primary  sur- 
faces of  the  dentine;  additional  bundles  run  obliquely  crosswise  in  the  layers 
of  dentine.  The  bundles  of  fibrilke,  from  2-3  /x  in  diameter,  appear  in  trans- 
verse sections  as  small  punctated  fields.  The  fibrillae  are  knit  together  by 
the  calcified  ground-substance,  in  which  the  lime-salts  are  deposited  in  the 
form  of  minute  spheres,  the  interstices  between  the  spherules  being  later 
filled  and  calcification  thus  completed.  When,  as  often  happens,  the  calci- 
fication is  imperfect,  irregular  clefts,  the  interglobular  spaces,  result.  These 
spaces  are  bounded  by  the  spherules,  or  de>itine-g/obu/es,  of  calcareous 
material  and  are  of  irregular  form  and  uncertain  extent,  being,  however, 
usually  largest  in  the  crown.  The 
junction  of  the  dentine  and  cement- 
um  is  always  marked  by  a  zone  of 
closely  placed  interglobular  spaces 
of  small  size;  under  low  magnifica- 
tion in  ground  sections  these  spaces 
appear  as  dark  granules,  hence  the 
zone  is  called  the  granule  layer 
of  Tomes. 

The  Cementum. — The  ce- 
mentum,  the  criista  petrosa  of  the 
older  writers,  forms  an  investment 
of  modified  bone  that  covers  the 
outer  surface  of  the  dentine  from 
the  neck  to  the  apex  of  the  tooth. 
Beginning  where  the  enamel  ends, 
or  overlapping  the  latter  to  a  slight 
extent,  the  cementum  gradually 
increases  in  thickness  until  over  the 
root,  especially  between  the  fangs 
of  the  molars,  it  forms  a  layer 
several  millimeters  thick.  The 
matrix  of  the  cementum  differs 
from  that  of  ordinary  bone  in  con- 
taining slightly  less  organic  matter 
and  a  greater  number  of  fibre-bundles  that  run  vertically  to  the  bone-lamellae. 
These  bundles  correspond  to  the  fibres  of  Sharpey  in  other  situations.  The 
lacunae  are  larger  than  those  of  ordinary  bone  and  the  canaliculi  are  unusually 
long  and  elaborate.  As  in  bone,  so  in  the  cementum  these  lymph-spaces 
contain  connective  tissue  elements,  the  cementum-cells.  Although  connect- 
ing with  one  another  by  means  of  the  canaliculi,  the  lacunae  seldom  commu- 
nicate with  the  dentinal  tubules,  the  latter  commonly  ending  in  loops  or 
blind  expansions.  The  outer  surface  of  the  cementum  is  intimately  attached 
to  the  surrounding  alveolar  periosteum,  the  so-called  pericementum,  since 
from  the  latter  the  cementum  is  derived.  Typical  Haversian  canals  are 
found  in  cementum  only  when  this  layer  is  hypertrophied. 

The  Alveolar  Periosteum. — The  periosteum  investing  the  jaws  also 
lines  the  sockets  receiving  the  roots  of  the  teeth,  which  are  by  this  means 
securely  held  in  place.      The  name,  pericementum  or  peridental  membrane. 


.^ 


X- 


^ 

'^J. 


^A^''> 


'^ 


'i 


\ 


^  ^ 


-it^' 


i^^^ ^^ 


Lacuna 


Fig.   175. — Ground  section  of  root  of  dried  tooth  in- 
cluding adjoining  dentine  and  cementum.     X  300. 


136 


NORMAL   HISTOLOGY. 


Dentine 


is  often  applied  to  this  part  of  the  periosteum,  which  lines  the  alveoli,  on  the 
one  hand,  and  covers  the  cementum  on  the  other  and  thereby  fulfils  the 
double  role  of  periosteum  and  root-membrane.  The  alveolar  periosteum  is 
made  up  of  tough  bundles  of  fibrous  tissue,  elastic  fibres  being  almost  want- 
ing, which  are  prolonged  as  the  penetrating  fibres  into  the  cementum,  on 
one  side,  and  as  the  fibres  of  Sharpey  into  the  alveolar  wall,  on  the  other. 
In  the  upper  part  of  the  root,  the  fibrous  bundles  are  almost  horizontal,  but 
towards  the  apex  they  are  more  oblique,  the  periosteum  here  losing  its  dense 
character  and  becoming  a  loose  connective  tissue  through  which  the  blood- 
vessels and  nerves  pass  to  the  root-canal  on  their  way  to  the  pulp.  At  the 
alveolar  border  the  pericementum  is  directly  continuous  with  the  stroma  of 
the  gum  and  -immediately  beneath  the  border  of  the  enamel  the  fibrous  bun- 
dles are  consolidated  into  a  dense  band,  the  ligmnentum  cimdare  dentis, 
which  still  further  aids  in  maintaining  the  firm  union  between  the  tooth  and 

the  alveolar  wall.  In  addition  to 
blood-vessels  and  nerves,  within  the 
pericementum  lie  irregular  groups 
of  epithelial  cells,  which  appear  as 
cords  or  networks  within  the  con- 
nective tissue  stroma.  The  groups 
are  the  remains  of  the  enamel-sheath 
(page  140),  which  surrounded  the 
young  tooth  during  its  early  devel- 
opment, and  have  been  mistaken 
for  glands. 

The  Pulp. — The  contents  of 
the  pulp-cavity  is  the  modified  meso- 
dermic  tissue  of  the  dental  papilla 
remaining  after  the  formation'  of  the 
dentine.  The  adult  pulp  consists 
chiefly  of  soft  highly  vascular  con- 
nective tissue,  containing  few  or  no 
elastic  fibres  but  many  irregularly 
distributed  cells.  The  general  type 
of  the  tissue  resembles  embryonal, 
both  in  its  fibrous  tissue  and  in  its 
cells,  the  latter  being  round,  oval  or 
stellate  with  long  processes.  The  peripheral  zone  of  the  pulp,  next  the  den- 
tine, is  of  especial  interest,  since  in  this  situation  lie  the  direct  descendants 
of  the  dentine-producing  cells,  the  odontoblasts.  These  are  tall  columnar 
cells,  arranged  vertically  to  the  surface  of  the  pulp,  about  25  p-  in  length  and 
5  !J.  in  breadth,  which  send  out  long  delicate  processes,  the  dentinal  fibres, 
into  the  dentinal  tubules  and  short  ones  into  the  pulp-tissue.  The  spaces 
between  the  bases  of  the  odontoblasts  are  occupied  by  smaller  cells,  less 
regularly  disposed  and  less  cylindrical  and  more  uncertain  in  form. 

The  blood-vessels  supplying  the  pulp  are  from  three  to  ten  small 
arteries  which  break  up  into  capillary  networks  soon  after  entering  the  pulp- 
cavity.  In  human  teeth  the  capillaries  usually  do  not  invade  the  layer  of 
odontoblasts.  The  larger  veins,  formed  by  thin-walled  radicles,  follow  the 
general  course  of  the  arteries.  Lymphatics  have  been  recently  demon- 
strated within  the  pulp  as  networks.  The  nerves  supplying  the  pulp-tissue 
are  numerous,  each  fang  receiving  a  main  stem  and  several  additional  smaller 
twigs,  which  in  a  general  way  accompany  the  blood-vessels.      They  include 


Blood- 
vessel 


V  iG.  176. — Section  of  periphery  of  pulp-tissue  of  young 
tooth.     X  175- 


TOOTH-DEVELOPMENT.  137 

both  medullated  and  nonmeduUated  fibres,  the  latter  being  sympathetic 
fibres  destined  for  the  walls  of  the  blood-vessels.  On  reaching  the  crown- 
pulp,  the  larger  twigs  are  replaced  by  finer  branches  that  subdivide  into 
many  fibres.  These,  on  reaching  the  periphery  of- the  pulp,  form  a  plexus 
beneath  the  laver  of  odontoblasts  from  which  nonmeduUated  axis-cylinders 
are  given  of5.  Some  of  these  end  beneath  the  odontoblasts  in  minute  nodular 
swellings;  others  penetrate  between  the  odontoblasts  and  terminate  in  pointed 
or  bulbous  free  endings.  There  is  no  trustworthy  evidence  that  the  nerves 
either  directly  join  the  odontoblasts  or  enter  the  dentine. 

DEVELOPMENT   OF   THE   TEETH. 

About  the  beginning  of  the  seventh  week  of  fcetal  life,  the  ectodermic 
epithelium  thickens  along  the  margins  of  the  oral  opening  and  forms  a  ridge 
of  proliferated  epithelium,  the  labio-dental  strand.  This  grows  into  the 
surrounding  mesoderm  and  divides  into  two  plates  w^hich,  while  continuous 
at  the  surface,  dixerge  almost  at  right-angles  at  the  deeper  plane.  The 
lateral  or  outer  plate  is  vertical  and  corresponds  to  the  plane  of  separation, 
effected  by  the  labial  fun'ow,  that  later  occurs  in  differentiating  the  lips 
from  the  tissues  forming  the  jaw.  The  median  or  inner  plate  grows  more 
horizontally  into  the  mesoderm  and  is  the  one  directly  concerned  in  the 
tooth-development;  for  this  reason  it  is  termed  the  denial  ledge. 

The  anlages  or  embryonic  rudiments  of  the  milk-teeth  are  indicated 
by  club-shaped  epithelial  outgrowths,  which  grow  from  the  deeper  surface 
of  the  dental  ledge  to  form  the  enamel-organs,  as  well  as  to  meet,  and 
later  cap,  the  mesodermic  elevations,  the  dental  papillcB.  The  bulbous 
end  of  the  epithelial  outgrowth  increases  rapidly  and  differentiates  into 
the  typical  three-layered  enamel-organ.  The  latter  is  attached  for  a 
time  to  the  dental  ledge  by  a  broad  strand  of  cells,  which  becomes  more 
and  more  reduced  until,  finally,  it  is  broken  and  the  enamel-organ  is 
isolated  from  the  oral  epithelium. 

The  Dental  Papilla. — This  structure  appears,  shortly  after  the 
club-shaped  enamel-organ  begins  to  expand  (Fig.  177,  C),  as  a  condensa- 
tion of  the  mesoderm  beneath  the  epithelial  outgrowth.  At  first  the 
papilla  consists  of  a  close  aggregation  of  small  round  proliferating  cells, 
but  later,  coincidently  with  the  differentiation  of  the  three  layers  of  the 
enamel-organ,  the  peripheral  cells  of  the  dental  papilla  become  elongated 
and  arranged  as  a  continuous  row  of  cylindrical  cells,  which  cover  the 
apical  portion  of  the  papilla  and  lie  beneath  the  enamel-organ.  These 
cylindrical  mesodermic  cells  are  the  odontoblasts,  the  active  agents  in 
the  production  of  the  dentine.  Where  engaged  in  this  process,  particularly 
over  the  summit  of  the  papilla,  the  cells  measure  from  35-50  11  in  length 
and  from  5-10  tJ.  in  breadth,  but  over  the  sides  of  the  papilla  they  gradually 
become  lower  and  less  characteristic,  until,  at  the  base,  they  resemble  the 
central  cells  of  the  papilla.  So  long  as  the  tooth  grows,  odontoblasts  are 
differentiated  in  the  vicinity  of  the  last-formed  part  of  the  root;  after  such 
differentiation  and  the  odontoblasts  engage  in  producing  dentine,  mitosis 
is  no  longer  observed  in  these  cells. 

The  formation  of  the  dentine  is  accomplished  through  the  agency 
of  the  odontoblasts  in  the  same  manner  as  the  osteoblasts  produce  the 
matrix  of  bone.  The  earliest  dentine  appears  as  a  thin  homogeneous 
stratum,  \\\q  menibrana  prcEforniativa,  which  overlies  the  tip  of  the  papilla 
and  underlies  the  enamel-organ.      This  layer  is  probably  absorbed  through 


I3S 


NORMAL    HISTOLOGY. 


the  influence  of  the  enamel.  The  denthie-matrix ,  deposited  through  the 
activity  of  the  odontoblasts,  is  for  a  time  uncalcified.  The  deposit  of  lime- 
salts  occurs  first  over  the  apex  of  the  papilla  and  next  the  enamel,  a  zone  of 
uncalcified  matrix  around  the  pulp-cavity  marking  the  youngest  dentine. 
The  calcareous  material  is  deposited  in  the  form  of  minute  spheres,  the  den- 
tine-2-lobides,  calcification  being 
completed  by  the  subsequent  fill-  ', 

ing   of    the   interglobular   clefts.  .       '^^-; 

When  for  any  reason  calcification  ' : 

is  incomplete,  these  clefts  remain  "^v    pentanedge 


Thickened 

oral- 
epitheliu:ii 


l.abio-dental 
strand  ~ 


Enamel- 
'  organ 


Dental  _ 
ledg-e" 


Oral 

epithelium* 


Outer 
layer 
enamel 
organ 


of 


Outer  layer- 

Middle 

layer 

Inner  layer 

of  enamel - 

organ 


Dental 
papilla 


Middle 
layer 


Anieloblasts 
-  Dentine 


_  Dental 
papilla 

Epithelial 
sheath 


Fig.  177. — Sections  hhowing  four  early  stages  of  toolh-developmeiit.     A,  B,  A  lb\  C,  D,  X  50. 

uninvaded  and  are  recognized  as  the  interglobular  spaces.  The  dentine- 
matrix  difiers  from  that  of  bone  in  being  produced  by  a  single  set  of  cells, 
while  the  bone-matrix  is  the  collective  work  of  relays  of  osteoblasts  which, 
while  contributing  their  increment,  become  imprisoned  in  the  lacunae  within 
the  matrix  that  they  have  formed.  In  human  dentine,  on  the  contrary,  the 
odontoblasts  remain  on  the  surface  and  only  exceptionally  become  enclosed 
within  the  dentine-matrix.  With  the  completion  of  dentine-production,  the 
odontoblasts  become  narrower  and  smaller  and  later  exhibit  evidences  of 


TOOTH-DEVELOPMENT. 


139 


impaired  vitality.      Their  dentinal  processes  likewise  grow  thinner  and  less 
flexible  and    gradually   assume    the    characteristics  of    the  dentinal    fibres. 
The    portions   of    the  dental    papilla  remaining  after  the  dentine  has  been 
completed  persist  as  the  definite  pulp-tissue,  receiving 
a  generous  ^'ascular  and  nervous  supply. 

The  Enamel-Organ. — The  end  of  the  ecto- 
dermic  epithelial  outgrowth,  which  early  marks  the 
position  of  the  future  tooth,  soon  broadens  and 
becomes  invaginated  to  form  the  young  enamel-organ 
that  overlies  the  top  of  the  mesodermic  dental  papilla 
(Fig.  177).  In  contrast  to  the  latter,  which  as  the 
pulp-tissue  partially  persists  as  a  permanent  structure, 
the  enamel-organ  is  only  transient  and  ultimately 
disappears.  When  fully  developed,  the  enamel-organ 
consists  of  three  principal  parts,  the  outer,  middle 
and  inner  layers.  Since  the  organ  is  converted  into 
a  cap  by  the  pushing  in  or  inv^agination  of  its  broader 
and  deeper  surface,  it  follows  that  the  outer  and  inner 
layers  are  directly  continuous  at  the  margin  of  the  invested  area.  The  outer 
layer  consists  of  flattened  epithelial  cells,  which  send  processes  into  the 
surrounding  vascular  connective  tissue  that  invests  the  tooth-germ  as  the 


Fig.  178. — Isolated  odonto- 
blasts from  tooth  of  new- 
born child.    X  300.    (Ebner.) 


,^-<3*:>^,^-- 


ai' 


«^P~1^^2^tes 


r^^^^-^-.-.       Intermediate  layer  of 

-----^••'  -■  enamel-organ 


Tx^-f^' 


'  ; 


my 


Ameloblasts 


Young  enamel  with 
Tomes's  processes 


Dentine 


„Last-formed  dentine 


(SS®* 


'^^ 


.Odontoblasts 


.  Embr>'onal  pulp-tissue 


Fig.  179. — Section  of  developing  tooth  through  junction  of  enamel  and  dentine.    X  400- 


dental  sac.  The  middle  layer,  conspicuous  by  reason  of  its  clear  loose 
texture,  consists  of  epithelial  elements  that  have  become  highly  modified 
in  consequence  of  an  enormous  distention  of  the  intercellular  clefts  by  fluid, 
the  epithelial  plates  in  this  manner  being  reduced  to  stellate  cells  connected 
by  delicate  processes.      The  inner  border  of  this  peculiar  area  constitutes 


140 


NORMAL    HISTOLOGY. 


a  transitional  zone  known  as  the  intermediate  layer.  This  is  best  marked 
over  the  upper  part  of  the  crown,  at  the  sides  thinning  out  and  entirely 
disappearing  at  the  margin  of  the  enamel-organ.  The  inner  layer  of  the 
enamel-organ  comprises  a  single  row  of  closely  set  tall  columnar  elements, 
the  enamel-cells,  also  called  adamanto blasts  or  ameloblasts,  through  whose 
active  agency  the  enamel  is  produced.  The  ameloblasts  are  best  developed 
over  the  top  of  the  dental  papilla,  where  they  measure  from  25-40  ix-  in 
length  and  from  4-7  11  in  breadth.  They  possess  oval  nuclei  that  usually 
lie  close  to  the  outer  ends  of  the  cells.  The  ameloblasts  are  united  by  a 
small  amount  of  cement-substance  and  defined  from  the  intermediate  layer 
by  a  distinct  border.  Over  the  sides  of  the  dental  papilla,  corresponding 
to  the  lateral  limit  of  the  future  crown,  the  ameloblasts  gradually  diminish 
in  height  until  they  are  replaced  by  low  cuboidal  cells  which,  at  the  margin 
of  the  enamel-organ,  are  continuous  with  the  epithelium  of  the  outer  layer. 
Preparatory  to  the  formation  of  the  dentine  of  the  root  of  the  tooth,  this 
margin  of  the  enamel-organ  grows  towards  the  base  of  the  elongating  dental 
papilla,  which  is  in  consequence  embraced  by  the  extension  of  the  enamel- 
organ.  This  investment  is  known  as  the  epithelial  sheath  (Fig.  177,  U), 
a  structure  of  importance  in  determining  the  form  of  the  tooth,  since  it  serves 
as  a  mould  in  which  the  dentine  is  deposited;  there  is,  however,  insufficient 
evidence  to  regard  the  epithelial  sheath  as  an  active  or  even  necessary  factor 
in  the  production  of  the  dentine. 

The  formation  of  the  enamel  results  from  the  activity  of  ectodermic 
epithelium  and  may  be  regarded  as  a  cuticular  development  carried  on  by  the 
ameloblasts.  The  initial  phase  in  the  production  of  the  enamel  is  the  appear- 
ance of  a  delicate  cuticular  zone  at  the  inner  end  of 
each  ameloblast;  this  fuses  with  the  zones  capping 
the  adjoining  cells  to  form  a  continuous  homogene- 
ous layer.  The  latter  differentiates  into  rod-like 
segments,  the  enamel-processes,  or  processes  of 
Tomes,  which  are  the  rudiments  of  the  enamel- 
prisms  and  the  interprismatic  substance.  The 
latter  gradually  decreases  in  amount  as  the  forma- 
tion of  the  enamel-columns  progresses,  since  the 
greater  part  of  this  intercolumnar  substance  is 
transformed  into  the  cortical  portion  of  the  enamel- 
prisms,  while  the  remainder  persists  as  the  meagre 
cement-substance  between  the  mature  prisms. 
The  enamel-processes  are  for  a  time  uncalcified, 
but  later  the  calcareous  material  is  deposited  as 
granules  and  spherules,  which  appear  first  in  the  axis  of  the  prisms.  The 
developing  enamel  increases  in  thickness  by  the  addition  of  the  increments 
formed  at  the  inner  ends  of  the  ameloblasts,  the  same  cells  sufficing  for  the 
production  of  the  entire  tissue.  The  earliest  formed  enamel  lies  in  apposition 
with  the  oldest  dentine,  the  youngest  enamel  immediately  beneath  the  amelo- 
blasts. The  enamel  is  deposited,  therefore,  from  within  outwards,  or  in  the 
reverse  of  the  direction  followed  by  the  growth  of  the  dentine.  The  oldest 
strata  of  both  substances  lie  in  contact;  the  youngest  on  the  outer  and 
inner  surfaces  of  the  tooth.  After  the  requisite  amount  of  enamel  has 
been  produced,  differentiation  into  prisms  ceases;  consequently  the  last- 
formed  enamel  remains  as  a  continuous  homogeneous  layer  which  invests 
the  free  surface  of  the  crown  and  constitutes  the  enamel-cuticle  or  membrane 
of  Nasmyth. 


Fig.  180.— Isolated  ameloblasts 
from  new-born  child,  a,  basal 
plate;  b,  cuticular  border;  c, 
processes  of  Tomes ;  rf,  homo- 
geneous mass  still  capping  proc- 
ess.    X  400.     {Ebner.) 


TOOTH-DEVELOPMENT.  141 

The  Tooth-Sac. — Coincidently  with  the  development  of  the  enamel- 
organ  and  the  growth  of  the  dental  papilla,  the  surrounding  mesoderm  dif- 
ferentiates into  a  connective  tissue  envelope  known  as  the  dental  or  tooth-sac. 
The  latter  not  only  closely  invests  the  enamel-organ,  but  is  intimately  related 
to  the  base  of  the  dental  papilla.  In  contrast  to  the  epithelial  enamel-organ 
which  is  entirely  without  blood-vessels,  the  inner  part  of  the  tooth-sac  is 
abundantly  provided  with  capillaries  and  is,  therefore,  an  important  source 
of  nutrition  for  the  growing  dental-germ.  The  part  of  the  sac  opposite  the 
root  of  the  young  tooth  at  first  is  prevented  from  coming  into  contact  with 
the  dentine  by  the  double  layer  interposed  by  the  epithelial  sheath.  This 
relation  continues  until  the  cementum  begins  to  develop,  when  the  vascular 
tissue  of  the  dental  sac  breaks  through  the  epithelial  sheath  to  reach  the 
outer  surface  of  the  dentine,  upon  which  the  cementum  is  deposited.  In 
consequence  of  this  invasion  the  epithelial  sheath  is  broken  up  into  small 
groups  or  nests  of  cells  that  persist  for  a  long  time  as  the  epithelial 
islands  encountered  within  the  fibrous  tissue  of  the  alveolar  periosteum, 
into  which  the  dental  sac  is  converted.  As  development  proceeds,  the 
tissue  of  the  tooth-sac  becomes  denser,  the  part  opposite  the  root  persisting 
as  the  pericementum,  while  the  superficial  part  blends  with  the  tissue 
forming  the  gum. 

The  formation  of  the  cementum  is  brought  about  through  the 
agency  of  mesodermic  tissue  in  a  manner  almost  identical  with  the  develop- 
ment of  subperiosteal  bone  (page  47),  the  cement-producing  cells,  the 
cementoblasts,  corresponding  to  osteoblasts,  and  like  them  bringing  about  a 
deposit  of  osseous  matrix  upon  the  osteogenetic  fibres  from  the  alveolar  peri- 
osteum. The  cementum  appears  first  in  the  vicinity  of  the  neck  of  the  tooth 
and  thence  progresses  towards  the  apex  of  the  root,  as  the  dentine  of  the  fang 
is  formed.  The  layer  of  cementum  is  thickest  at  the  apex,  which  it  invests 
except  where  the  canal  or  canals  remain  for  the  blood-vessels  and  nerves 
that  pass  to  the  pulp-cavity. 

Provision  for  the  development  of  the  permanent  teeth  is  made  by 
the  early  differentiation  of  a  second  set  of  dental  rudiments  during  the 
growth  of  the  first.  This  includes  the  outgrowth  of  the  enamel-organs  of 
second  dentition  from  the  dental  ledge  and  the  subsequent  appearance  of 
new  dental  papillae  from  the  mesoderm.  The  enamel-organ  for  the  first  per- 
manent molar  appears  about  the  seventeenth  week  of  foetal  life  and  is  soon 
followed  by  the  corresponding  dental  papilla.  The  germs  of  the  permanent 
incisors  and  canines,  including  the  papillae,  are  formed  about  the  twenty-ninth 
week  and  those  for  the  premolars  about  one  month  later.  The  enamel- 
organ  of  the  second  permanent  molar  appears  about  four  months  after  birth 
and  the  papilla  about  two  months  later,  while  the  enamel-sac  for  the  third 
molar,  which  forms  about  the  third  year,  precedes  its  papilla  by  almost  two 
years.  It  is  evident,  therefore,  that  the  development  of  these  teeth  proceeds 
very  slowly,  the  embryonal  structures  being  present  years  before  the  eruption 
of  the  permanent  teeth.  The  presence  of  the  milk  teeth  and  of  the  germs  of 
the  permanent  ones  results  in  excessive  crowding  in  the  jaws  during  the  fifth 
year.  In  order  to  accommodate  the  representatives  of  both  sets,  the  crowns 
of  the  permanent  teeth  press  between  and  against  the  roots  of  the  milk  teeth, 
which  then  undergo  absorption.  The  latter  process  is  effected  by  connective 
tissue  cells,  the  odontoclasts,  in  a  manner  similar  to  that  bv  which  bone  is 
removed  by  the  osteoclasts.  In  consequence,  before  the  temporary  tooth  is 
displaced  it  often  is  reduced  to  little  more  than  the  crown.  Not  until  some 
time  after  eruption  are  the  roots  of  the  permanent  teeth  fully  formed. 


142  NORMAL    HISTOLOGY. 


THE   TONGUE. 

The  tongue  is  essentially  a  complex  mass  of  striped  muscle,  the  free 
surfaces  of  which  are  covered  by  an  extension  of  the  mucous  membrane 
lining  the  mouth  and  the  pharynx. 

The  muscles  of  the  tongue  include  two  groups,  the  extrinsic  and  the 
intrinsic.  The  former  (the  genio-glossus,  the  hyo-glossus,  the  stylo-glossus 
and  the  palato-glossus)  are  all  paired  and  extend  from  the  skull  or  the  hyoid 
bone  to  the  tongue;  the  latter  comprise  the  particular  muscle,  the  lingualis, 
forming  the  chief  mass  of  the  organ.  The  tongue  is  incompletely  divided 
into  symmetrical  halves  by  a  vertical  partition  of  dense  connective  tissue,  the 
septum  lingu(z,  which  extends  from  the  hyoid  bone  behind  to  the  tip  of  the 
tongue  but  fades  away  before  reaching  the  apex.  It  is  much  better  developed 
in  the  middle  third  of  the  tongue  than  at  the  ends,  but  even  here  the  septum 

Longitudinal  fibres 


^ -Vertical 


fibres 


Glands  —    ,      ~~  &-^  '  , 

^  »  i      '  \ 

Portion  of  sublingual  gland  Septum        Genio-glossus     Hyo-glossus 

Fig.  i8i. — Transverse  section  of  child's  tongue,  through  middle  third.    X  3. 

falls  short  of  the  dorsal  mucous  membrane  by  a  few  millimeters.  On  viewing 
with  low  magnification  a  cross-section  of  the  tongue  at  its  middle  third 
(Fig.  181  j,  the  intricate  feltwork  of  muscle-fibres  is  seen  to  comprise  fibres 
running  in  three  general  directions — longitudinal,  vertical  and  transverse. 
The  longitudinal  fibres  appear  transversely  sectioned  and  form  a  well-marked 
superficial  or  cortical  layer,  some  5  mm.  thick,  immediately  beneath  the 
scanty  submucous  tissue  covering  the  dorsum.  These  fibres  include  the 
principal  part  of  the  lingualis  muscle,  supplemented  by  fibres  from  the  stylo- 
glossus. The  vertical  fibres,  most  conspicuous  as  the  deeply  placed  and 
obliquely  cut  masses  of  the  genio-glossus  on  either  side  of  the  septum, 
radiate  towards  the  dorsal  surface,  where  the  lingual  bundles  end  in  the  sub- 
mucous la}'er.  The  transverse  fibres  are  entirely  from  the  lingualis,  with  the 
exception  of  those  contributed  by  the  palato-glossus.  They  arise  from  the 
septum  and  interlace  with  the  vertical  and  longitudinal  bundles;  on  approach- 
ing the  mucosa,  they  break  up  into  strands  which  find  their  way  between 
the  superficial  longitudinal  fibres  to  a  submucous  insertion.  Branching  is  a 
peculiarity  exhibited  by  many  muscle-fibres  that  end  in  the  submucosa. 

The  mucous  membrane  of  the  tongue  corresponds  in  general  structure 
with  that  lining  the  adjacent  surfaces  of  the  mouth  and  pharynx  in  consisting 
of  the  epithelium,  the  tunica  propria  and  the  submucous  layer.      Over  the 


THE   TONGUE. 


143 


sides  and  under  surface  of  the  tongue  it  is  thin  and  smooth,  with  small 
papillcc  towards  the  tip;  on  the  dorsum  the  mucous  membrane  is  greatly 
modified  and  presents  characteristic  appearances.  The  dorsal  surface,  more- 
over, includes  two  areas,  \S\&  papillary  and  the  lymphoid,  which  exhibit  very 
different  details. 

The  papillary  area  comprises  the  anterior  two  thirds,  the  lymphoid 
the  posterior  third.  In  the  infant's  tongue  the  junction  of  these  areas  is 
marked  by  a  V-shaped  groove,  the  siilais  tcrminalis,  but  this  disappears 
and  later  the  boundary  is  indicated  by  the  conspicuous  row  of  circumvallate 
papillae.  The  anterior  area  is  everywhere  beset  with  elevations,  the  papillae, 
which  are  of  three  varieties — the  filiform,  the  fungiform  arid  the  circumval- 
late.     The  JiliforDi  or  conical  papillce,  so  abundant  as  to  impart  a  velvety 


Epithelium  — ,fi^  giiS 


Tunica  propria ^Cj/,^ 


(fi 


^^^^^^S£i|iS^ 


V~ —  Connective  tissue 


*T,      '  Epitlnelium 


Blood-vessel 


Muscle-fibres 


F;g.  1S2. — Section  of  lingual  mucous  membrane  showing  filiform  papilla?.     X  75- 

appearance  to  the  tongue,  are  conical  or  cylindrical  elevations  of  the  tunica 
propria,  composed  of  fibrous  connective  tissue  with  many  elastic  fibres  and 
covered  by  a  thick  stratum  of  epithelium.  The  surface  of  the  connective 
tissue  core  bears  a  number  of  small  secondary  papillae,  which,  however,  do 
not  model  the  free  surface  of  the  mucous  membrane.  The  filiform  papillae 
vary  from  .5-2.5  mm.  in  height  and  often  end  in  brush-like  strands  of.  horny 
epithelial  cells.  The  fiino^ifonii  papilla;  are  far  less  numerous  than  the  fili- 
form and  appear  during  life  as  red  points,  chiefly  near  the  margins  of  the 
tongue,  in  consequence  of  their  thinner  epithelium.  As  implied  by  their 
name,  they  are  more  or  less  mushroom-like  in  form  and  vary  from  .5-1.5  mm. 
in  height.  The  connective  tissue  core  is  beset  with  a  number  of  secondary 
papillae,  over  which  stretches  a  smooth  layer  of  epithelium.  The  latter 
contains  occasional  taste-buds.  The  circiivivallate  papilla;,  the  most  con- 
spicuous of  these  elevations,  usually  number  nine  or  ten,  from  six  to  sixteen 
being  the  extremes.  They  are  disposed  as  an  irregular  V,  with  the  apex 
of  the  group  directed  backwards.  These  elevations,  from  1-1.5  mm.  high 
and  from   2-3.5  ''^ti"'''-  broad,  consist  essentially  of  a  fungiform  papilla   sur- 


144 


NORMAL   HISTOLOGY. 


rounded  by  a  shallow  groove  bounded  externally  by  a  low  annular  walL 
Their  chief  interest  is  their  being  the  principal  seats  of  the  taste-buds 
(page  385),  which  are  lodged  within  the  epithelium  lining  the  mesial  wall 


Filiform  papilla* 


f 


Surface  epithelium 
covering  fungiform 
'^^\\      papilla 

Projections  of  tunica 
propria  constituting 
;      basis  of  papilla 


;U 


-iiiW. 


Connective  tissue 
•  stroma  of  mucous 
membrane 


A^ 


.  Muscular  tissue  of 
tongue 


Fig.  183. — Section  of  lingual  mucous  membrane,  showing  fungiform  papilla.    X  .75. 

of  the  groove.     Into  the  bottom  of  this  groove  open  the  ducts  of  the  serous 
lingual  glands.     At  the  edges  of  the  tongue,  on  each  side  just  in  front  of  the 


Epithelium^ 


^WK  ' 


/ 


Taste-bud 


Annular  wall  • 


.  Gland-duct 


Central  part  0% 

papilla — 

connective  tissue 


Serous  gland 


Muscle  fibres' 


'  ii  ■^;*U'vC 


Fig.  184. — Section  across  circumvallate  papilla  from  child's  tongue,  showing  central  part  and  encircling 

wall.     X  45- 

anterior  palatine  pillar,  may  be  seen  a  series  of  small  transverse  parallel 
ridges,  the  papillce  foliates.  This  structure,  varyingly  pronounced  in  man 
but  well  developed  in  the  rodents,  is  of  interest  on  account  of  the  many  taste- 
buds  which  it  often  contains. 


THE   TONGUE. 


145 


The  posterior  or  lymphoid  area,  the  part  of  the  dorsum  behind  the 
circumvallate  papillae,  presents  a  striking  contrast  to  the  anterior  two  thirds 
of  the  mucous  membrane.  The  surface  is  thrown  into  uncertain  rounded 
elevations,  which,  while  smooth  and  devoid  of  papillae,  impart  to  this  portion 
of  the  tongue  an  uneven  and  mammillated  appearance  that  is  further  accen- 
tuated by  numerous  minute  pits.  The  latter  lead  into  small  crypts,  each 
lined  by  stratified  epithelium  continued  from  the  surface  and  surrounded  by 
a  zone  of  lymphoid  tissue.  The  latter  contains  a  number  of  lymph-nodules, 
with  germ-centres,  blended  together  by  the  intervening  diffuse  lymphoid 
tissue  and  lodged  in  the  tunica  propria.  These  spherical  masses,  X\\^  follicidi 
ling7iales,  resemble  in  miniature  the  lymphoid  organs  found  on  the  side-walls 
of  the  oro-pharynx  and,  hence,  are  often  termed  collectively  \S\&  lingual  tonsil. 

The  glands  of  the  tongue  include  both  mucous  and  serous  varieties, 
which  are  distributed  in  three  groups:  (i)  the  serous  glands,  (2)  the  mucous 
glands  and  (3)  the  anterior  sero-mucous  glands.  The  serous  glands  are 
small  and  occur  in  the  vicinity  of   the  circumvallate  and  foliate  papillae, 


Glands 


Fig.  185. 


-Section  from  posterior  third  of  child's  tongue,  showing  lymphoid  tissue  constituting  a  part  of 
the  lingual  tonsil.     X  30. 


occupying  the  deeper  part  of  the  tunica  propria,  with  some  alveoli  between 
the  subjacent  muscle-bundles.  Their  ducts  lead  through  the  mucosa  and 
open  preferably  at  the  bottom  of  the  furrows  along  which  the  taste-buds  are 
lodged.  They  belong  to  the  tubo-alveolar  group,  with  the  tubular  type  of 
the  ultimate  compartments  pronounced.  The  gland-cells  are  somewhat  pyram- 
idal, rest  upon  a  basement  membrane  of  great  delicacy,  and  secrete  a  thin, 
watery,  albuminous  fiuid.  The  mncoics  glands  are  found  just  in  advance  of 
the  more  median  circumvallate  papillae,  along  the  margins  of  the  tongue,  and 
scattered  through  the  lymphoid  area,  especially  towards  the  root.  They  are 
tubo-alveolar  in  type  and  among  the  examples  of  pure  mucous  glands. 
Their  viscid,  mucin-containing  secretion  is  produced  by  the  cylindrical 
gland-cells  and  carried  off  by  ducts,  which  are  lined  with  columnar  epithe- 
lium and  open  on  the  free  surface  or,  not  infrequently,  into  the  lymphoid 
crypts.  The  anterior  lingual  glands,  or  glands  of  Nuhn,  form  two  elon- 
gated groups,  15-20  mm.  long  and  7-9  mm.  wide,  which  lie  near  the  tip  of 
the  tongue,  on  either  side  of  the  mid-line.  Both  serous  cells  and  mucous 
alveoli  occur,  hence  they  belong  to  the  mixed  mucous  and  tubo-alveolar  type 
and  possess  demilunes.      They  open  on  the  under  surface  of  the  tongue. 

The  blood-vessels  of  the  tongue  include,  in  addition  to  the  branches 
of  the  lingual  artery  that  break  up  into  capillary  networks  supplying  the 
muscles,  a  vascular  meshwork  within  the  mucous  membrane  from  which 
twigs  ascend  to  the  papillae.      Each  of  the  latter  contains  a  tuft  of  elongated 


146 


NORMAL   HISTOLOGY. 


capillary  loops  (Fig.  186)  which  occupies  the  connective  tissue  stroma, 
including  that  of  the  secondary  papillae.  Other  twigs  pass  to  the  glands  and 
end  in  capillary  networks  that  surround  the  alveoli;  still  others  provide 
capillaries  which  ramify  within  the  aggregations  of  lymphoid  tissue.  The 
lymphatics  of  the  tongue  constitute  two  groups,  a  superficial  within  the 
mucous  and  submucous  layers  and  a  deep  one  within  the  musculature.  The 
submucous  vessels  arise  from  the  rich  network  of  lymph-channels  within  the 


IP.X  iV  \  ^  ''  i.  "1 


Epithelium  covering 
filiform  papilla; 


Capillary  loops 
within  connective- 
tissue  basis  of 
papillae 


Mucous  membrane 


Muscular  tissue 


Fig.  186. — Injected  mucous  membrane  from  upper  surface  of  tongue.     X  6°. 

dorsal  mucosa,  the  apical  network  being  especially  close.  The  lymph- 
nodules  of  the  posterior  area  are  surrounded  by  meshes  of  lymphatics. 

The  nerves  of  the  tongue  include  three  sets  of  fibres  concerned  in 
conveying  the  impulses  for  common  sensation,  taste  and  motion.  Those  for 
common  sensation,  derived  from  the  trigeminus  and  glossopharyngeal  nerves, 
end  in  the  lingual  mucous  membrane  in  the  manner  already  described  for  the 
oral  mucosa  (page  130),  the  papillae  receiving  fibres  that  confer  great  sen- 
sibility. The  gustatory  impulses  are  collected  by  the  fibres  from  the  chorda 
tympani  and  the  glossopharyngeal,  which  are  distributed  to  the  anterior 
two  thirds  and  the  posterior  third  respectively.  Such  fibres  end  in  close 
relation  with  the  receptive  neuroepithelial  cells  within  the  taste-buds  (page 
385).  The  motor  fibres  are  from  the  hypoglossal  nerve.  As  in  other  locali- 
ties, so  here,  nonmeduUated  sympathetic  fibres,  destined  for  the  blood-vessels 
and  glands,  are  included  in  many  nerve-trunks  along  with  medullated  fibres. 

The  Oral  Glands. — In  addition  to  the  large  salivary  glands  which, 
although  pouring  their  secretion  into  the  oral  cavity,  are  situated  outside  of 
the  immediate  walls  of  the  mouth,  certain  groups  of  glands,  for  the  most 
part  insignificant  in  size,  lie  within  the  oral  wall  and  contribute  secretions 


THE    SALIVARY    GLANDS.  147 

serving  for  the  lubrication  of  the  oral  mucous  membrane.  Such  oral  glands 
include  the  labial,  within  the  lips,  the  buccal  and  the  molar,  within  the 
cheeks.  The  labial  glands  are  very  numerous  and  constitute  an  almost 
unbroken  zone,  something  over  a  centimeter  in  width,  that  surroiuids  the 
oral  cleft  just  inside  the  margin  of  the  red  lip-area.  They  are  alveo-tubular 
in  type,  lie  within  the  submucous  layer,  between  the  mucous  membrane  and 
the  muscle,  and  belong  to  the  mixed  mucous  glands.  The  gland-cells, 
although  chiefly  mucous  in  character,  include  limited  peripheral  groups  of 
serous  cells  arranged  as  demilunes  beneath  the  basement  membrane.  The 
buccal  glands,  smaller  and  more  scattered,  occupy  the  submucosa  beneath 
the  buccinator  muscle.  They  correspond  in  structure  with  the  labial  glands. 
The  same  is  true  of  the  molar  glands,  several  small  groups  on  the  outer 
surface  of  the  buccinator  muscle,  whose  ducts  pierce  the  cheek. 

THE   SALIVARY    GLANDS. 

The  salivary  glands  include  the  parotid,  the  submaxillary  and  the  sub- 
lingual,  all  organs  of  some  size,  and  elaborate  secretions  poured  into  the 
oral  cavity  to  assist  in  preparing  the  food  for  deglutition  and  further  chemi- 


Interlobular  duct 
Artery 

Vein 


^. 


Intralobular 
duct 


Duct 


Interlobular 
septum 


Duct 


-•i^   V>:'; 


"«»^^ 


Fig.  187. — Section  of  small  lobule  of  parotid  gland.     X  7". 

cal  change.  For  the  latter  purpose,  the  parotid  gland  is  of  most  importance 
and  in  all  animals  maintains  its  character  as  a  true  (serous)  salivary  gland. 
The  others,  the  submaxillary  and  sublingual,  are  variably  mixed  glands,  the 
former  sometimes  and  the  latter  never  approaching  the  pure  serous  type. 


148 


NORMAL    HISTOLOGY. 


The  Parotid  Gland. — This,  the  largest  of  the  salivary  glands,  lies 
between  the  upper  part  of  the  ramus  of  the  lower  jaw,  which  it  overlaps 
both  within  and  without,  and  the  external  ear.  It  is  invested  by  a  strong 
fibrous  sheath,  continuous  with  the  cervical  fascia.  The  gland  is  subdivided 
into  many  lobules  by  septa  of  dense  fibro-elastic  tissue  and,  hence,  possesses 
considerable  toughness.  The  parotid  consists  entirely  of  serous  alveoli, 
although  mucus-producing  acini  may  occur  in  the  accessory  lobules  along 
the  main  (Stenson's)  duct.  ^\\&  primary  lobules  are  made  up  of  alveoli, 
lined  with  pyramidal  glandular  epithelium  whose  appearance  changes  with 
functional  activity.  When  at  rest,  the  cells  are  filled  with  minute  secretion- 
granules;  the  latter,  however,  are  sensitive  to  reagents,   often  undergoing 


Intermediate  duct 


Interlobular  duct 


Tubular  alveolus 


Alveolar  lumen 


Connective  tissue 


Fig.  188. — Section  of  parotid  gland,  showing  serous  alveoli.    X  270. 

partial  or  complete  solution.  Hence,  the  reticulated  appearance  of  the  cyto- 
plasm frequently  observed  in  the  gland-cells  after  fixation.  The  spherical 
nuclei  occupy  the  middle  of  the  serous  cells. 

The  duct-system  begins  at  the  alveoli  as  the  intermediate  tubules,  which 
in  the  parotid  are  relatively  long  and  narrow  and  lined  with  low  or  flattened 
cells  continuous  with  the  alveolar  epithelium.  Secretion-canaliculi  extend 
between  the  gland-cells  part  way  to  the  basement  membrane.  The  intra- 
lobular tubules,  of  larger  diameter  (35  /v.)  than  either  the  immediately  pre- 
ceding or  succeeding  segments  of  the  duct,  are  lined  with  a  single  layer  of 
columnar  cells  that  exhibit  differentiation  into  an  inner  zone,  finely  granular 
and  containing  the  nucleus,  and  an  outer  or  basal  zone,  next  the  basement 
membrane  and  displaying  a  faint  longitudinal  striation  or  "rods."  The 
interlobular  and  interlobar  ducts  gradually  increase  in  diameter  and,  for  the 
most  part,  possess  a  single  layer  of  columnar  cells;  in  the  larger  canals,  how- 
ever, this  may  be  reinforced  by  an  additional  imperfect  row  of  small  cells. 
The  columnar  cells  extend  to  near  the  termination  of  the  main  excretory 
duct,  where  they  give  place  to  the  stratified  squamous  epithelium  continued 
into  the  canal  from  the  oral  mucous  membrane.  The  wall  of  the  parotid  or 
Stenson  s  duct,  in  addition  to  the  epithelium,  consists  of  fibrous  tissue,  mixed 
with  many  elastic  fibres  and  a  few  bundles  of  unstriped  muscle. 


THE   SUBMAXILLARY    GLAND. 


149 


The  Submaxillary  Gland. — This  organ,  intermediate  in  size  between 
the  parotid  and  sublingual  glands,  lies  largely  under  cover  of  the  lower  jaw, 
invested  by  a  fibrous  capsule  derived  from  the  cervical  fascia.  It  consists  of 
two  parts,  the  superficial,  resting  on  the  outer  surface  of  the  mylo-hyoid 
muscle,  and  the  deeper,  which  winds  around  the  posterior  border  of  the  last 
named  muscle  and  extends  forwards  as  a  tongue-like  process,  between  the 
mylo-hvoid  and  the  hyo-glossus  muscles  almost  as  far  as  the  sublingual 
gland,  beneath  the  mucous  membrane  of  the  fioor  of  the  mouth. 

The  submaxillary  gland  differs  in  structure  from  the  parotid  in  possessing 
both  serous  and  mucous  alveoli,  the  serous  forming  appro .ximately  one  fifth 
of  the  entire  gland-tissue.  The  alveoli  containing  serous  cells  resemble 
those  of  the  parotid,  during  rest  being  filled  with  cells  loaded  with  minute 


Mucous  alveoli 


Serous  alveoli 
Fig.  189. — Section  of  submaxillar'  gland,  showing  serous  and  mucous  alveoli.    X  270. 

secretion  granules.  These  cells  often  exhibit  a  differentiation  into  an  inner 
granular  and  an  outer  almost  granule-free  zone.  The  mucous  alveoli  are 
somewhat  larger  than  the  serous  ones  and  contain  chiefly  mucus-secreting 
cells,  although  limited  groups  of  serous-  cells  are  present  as  demilunes. 
Intermediate  tubules  connect  alveoli  of  both  kinds  with  the  intralobular 
ducts,  those  from  the  mucous  alveoli  being  shorter  and  less  branched  than 
those  from  the  serous  acini.  The  latter  are  lined  with  low  cuboidal  cells, 
which  pass  gradually  into  the  gland-cells  of  the  alveoli,  in  contrast  to  the 
abrupt  transition  in  the  tubules  leading  to  the  mucous  acini.  The  cells  lin- 
ing the  intralobular  ducts  exhibit  the  striation  seen  in  the  corresponding 
part  of  the  parotid,  this  rod-epithelium  sometimes  containing  yellowish  pig- 
ment. The  interlobular  and  interlobar  ducts  resemble  those  of  the  parotid 
gland.  The  main  excretory  or  Wharton^  s  duct  possesses,  in  addition  to  its 
fibrous  tissue  and  a  subepithelial  layer  of  elastic  fibres,  feebly  developed 
bundles  of  longitudinally  disposed  unstriped  muscle.  Goblet-cells  appear 
among  the  columnar  epithelium  lining  the  deeper  parts  of  the  duct. 

The  Sublingual  Gland. — This,  the  smallest  of  the  salivary  glands, 
underlies  the  mucous  membrane  of  the  floor  of  the  mouth,  through  which  it 
shows  as  an  oval  elevation  at  the  side  of  the  median  fold,  the  frenulum.     The 


I50 


NORMAL  HISTOLOGY. 


gland  does  not  possess  a  distinct  capsule  and  consists  of  an  aggregation  of 
separate  glands,  each  of  which  has  its  independent  duct,  rather  than  a  con- 
solidated single  organ. 

The  subhngual  gland  belongs  to  the  mixed  mucous  type  and  possesses 
a  curtailed  duct-system,  in  which  distinctive  intralobular  and  intermediate 
ducts  are  wanting.  The  interlobular  ducts  subdivide  into  smaller  canals  that 
enter  the  primary  lobules  and  give  off  wider  passages  lined  with  cuboidal 
epithelium.  Towards  the  distal  ends  of  these  terminal  canals  the  mucous 
gland-cells  appear,  at  first  isolated  or  in  groups,  until  they  constitute  the 
entire  lining  of  the  passage.  The  condition  of  the  alveoli,  as  regards  the 
mucus-producing  cells,  varies  even  in  the  same  lobule.  Sometimes  an  entire 
primary  lobule  is  composed  of  alveoli  filled  with  mucous  cells;  at  other  times 


Duct 


Demilunes  of- 
serous  cells 


Fig.  190. — Section  of  sublingual  gland,  showing  serous  cells  grouped  as  demilunes.     X  270. 

empty  and  engorged  alveoli  alternate,  or  the  depleted  acini  may  predominate. 
The  demilunes  of  serous  cells  are  also  uncertain,  since  these  may  be  absent  or 
present  in  considerable  numbers  and  of  large  size.  The  relatively  wide  lumen 
of  the  alveoli  and  the  more  pronounced  reticulated  appearance  of  the  gland- 
cells  distinguish  the  exhausted  sublingual  gland  from  the  parotid  under  like 
conditions. 

The  normal  secretions  of  the  salivary  and  oral  glands  contain  no  formed 
elements,  although  granules  and  cellular  remains  may  be  present.  The 
spherical  so-called  salivary  corpuscles,  which  occur  in  varying  numbers  in 
the  mixed  oral  secretion,  have  no  relation  to  the  salivary  glands,  since  they 
are  only  modified  lymphocytes  which  have  escaped  from  the  lymphoid  tissue 
of  the  faucial  and  lingual  tonsils.  On  entering  the  mouth,  these  cells  are 
affected  by  the  saliva  and  become  greatly  swollen,  the  granular  remains  of 
their  cytoplasm  exhibiting  molecular  motion  to  a  marked  degree. 

The  blood-vessels  of  the  salivary  and  larger  oral  glands  follow  the 
same  general  arrangement.  The  larger  arteries  accompany  the  ducts  in 
their  course  within  the  interlobular  connective  tissue,  giving  off  twigs  which 
supply  the  walls  of  the  excretory  passages.      From  the  interlobular  vessels. 


THE   PALATE.  151 

branches  enter  the  connective  tissue  between  the  primary  divisions  of 
the  glandular  tissue  and  eventually  break  up  into  close  capillary  networks 
which  surround  the  individual  alveoli,  the  basement  membrane  and  the 
endothelium  alone  separating  the  blood-stream  and  the  gland-cells.  The 
veins  maintain  the  general  path  of  the  arteries.  The  definite  lymphatics 
are  limited  to  the  interlobular  tissue,  usually  accompanying  the  ducts. 
The  perialveolar  clefts  between  the  bundles  of  the  fibrous  framework 
represent  the  interlobular  lymph-channels,  which  are  tributary  to  the 
lymphatic  vessels  between  the  lobules.  The  nerves  supplying  the  salivary 
glands  form  plexuses  within  the  interlobular  tissue  and  include  both 
meduUated  and  nonmeduUated  fibres.  The  latter,  destined  chiefly  for 
the  blood-vessels  and  gland-tissue,  are  in  part  the  axones  of  sympathetic 
neurones,  whose  cell-bodies  are  the  ganglion-cells  collected  in  microscopic 
groups  along  the  walls  of  the  larger  ducts.  On  reaching  the  alveoli,  the 
nonmeduUated  fibres  form  epilemmar  plexuses  on  the  outer  surface  of  the 
basement  membrane,  from  which  delicate  fibrils  pierce  the  membrane  to  end 
as  varicose  hypolemmar  threads  between  the  gland-cells. 

THE    PALATE. 

The  hard  palate  forms  the  roof  of  the  mouth,  and  is  bounded  laterally 
by  the  alveolar  border  of  the  upper  jaw.  The  mucous  membrane  is  united 
firmly  to  the  periosteum  covering  the  roof-bones  by  the  dense  connective 
tissue  of  the  submucous  layer.  Near  the  front  border  of  the  area,  the 
mucous  membrane  is  thrown  into  a  series  of  irregular  ridges,  the  palatal 
riigce,  over  which  the  secondary  papilla  of  the  tunica  propria  are  well 
marked.  On  each  side  of  an  oval  median  area,  the  submucosa  is  occupied 
by  an  almost  continuous  layer  of  palatal  glands,  composed  of  small  groups 
of  mucous  alveoli,  whose  ducts  open  on  the  surface  as  minute  scattered 
orifices. 

The  soft  palate  is  essentially  a  fold  of  mucous  membrane,  enclosing 
muscles,  tendinous  expansions  and  glands,  continued  backwards  and  down- 
wards from  the  hard  palate  to  form  the  mobile  arched  partition  between 
the  nasal  and  oral  subdivisions  of  the  pharynx.  The  free  border  is 
prolonged  in  a  median  conical  prominence,  the  uzmla,  which  breaks  the 
general  curve  of  the  border  into  a  double  arch.  The  soft  palate  includes 
four  general  layers,  which,  from  above  downwards,  are:  (i)  ^\\^ pharyngeal 
vincoiis  membrane,  which  for  some  distance  above  the  free  edge  of  the 
palate  corresponds  with  the  oral  mucosa  in  possessing  stratified  squamous 
epithelium  and  a  layer  of  elastic  fibres  in  the  deeper  margin  of  the  tunica 
propria.  Higher,  but  at  a  variable  distance  from  the  free  border,  the  mucosa 
changes  and  assumes  the  characteristics  of  the  respiratory  mucous  mem- 
brane. (2)  The  Jibro-rnuscular  layer,  which  comprises  the  expansions  of 
the  tendons  of  the  tensor  palati  muscles  and  a  complex  of  striped  muscle 
bundles.  (3)  The  glandular  layer,  which  is  in  places  5-6  mm.  thick  and 
continuous  with  the  glands  of  the  hard  palate.  The  glands  are  examples 
of  the  pure  mucous  variety,  and  so  closely  set  that  they  form  a  layer 
broken  only  in  the  mid-line  near  the  hard  palate  by  a  fibro-muscular 
septum.  They  mostly  end  near  the  free  border  of  the  palate  in  the  vicinity 
of  the  base  of  the  uvula,  but  some  are  continued  into  this  projection,  almost 
to  its  tip,  as  a  cylindrical  tract  of  glands  through  and  about  which  run 
the  fibres  of  the  azygos  uvulae  muscle.  (4)  The  oral  mucous  membrane, 
which,    although   strictly    belonging  for  the  most  part  to   the    oro-pharynx. 


152 


NORMAL    HISTOLOGY. 


resembles  the  general  lining  of  the  mouth.  The  submucous  tissue  of  this 
surface  contains  a  variable  amount  of  fat,  which  is  wanting  in  the  correspond- 
ing position  on  the  upper  aspect  of  the  palate.      Scattered  small  taste-buds 


Glands 


I. 


-^'.''Wt' 


Glands 


i 


Aponeurotic  tissue 


Pharyngeal 
mucous 

membrane 

Obliquely  cut 
muscles 


ral  mucous  membrane 


Fig.  191. — Lateral  sagittal  section  of  soft  palate.    X  15. 


are  occasionally  encountered  within  the  epithelium  of  the  lower  surface. 
The  ultimate  distribution  of  the  blood-vessels  and  nerves  of  the  palatine 
mucosa  follows  the  general  plan  of  the  oral  mucous  membrane. 


THE    PHARYNX. 

The  pharynx  or  throat  is  a  musculo-fibrous  sac,  lined  with  mucous 
membrane,  that  extends  from  the  base  of  the  skull  to  the  level  of  the  upper 
border  of  the  seventh  cervical  vertebra.  It  communicates  with  the  Eusta- 
chian tubes,  the  nasal  fossae,  the  oral  cavity  and  the  larynx,  and  is  continuous 
below  with  the  oesophagus.  The  walls  of  the  pharynx  comprise  three  gen- 
eral strata:  (i)  the  muscular  layer,  made  up  of  the  striated  fibres  of  the 
pharyngeal  constrictors  and  the  associated  muscles;  (2)  \hQ. fibrous  layer,  a, 
membranous  framework  of  dense  fibro-elastic  tissue,  strong  above  but 
weaker  below  and  continued  into  the  oesophagus;  and  (3)  the  viucous 
membrane,  which,  with  the  submucous  layer,  lines  all  parts  of  the  pharynx 
and  presents  striking  local  modifications. 

Within  the  naso-pharynx,  that  division  lying  above  the  level  of  the  soft 
palate,  the  mucous  membrane  mostly  resembles  that  of  the  adjoining  respira- 
tory part  of  the  nasal  fossae,  being  clothed  with  stratified  ciliated  epithelium 
and  containing  many  small  mixed  (sero-mucous)  tubo-alveolar  glands.  Over 
portions  of  the  posterior  wall  of  the  naso-pharynx,  as  well  as  over  the  uvula  and 
neighboring  region  of  the  upper  surface  of  the  soft  palate,  the  epithelium  is 


THE   PHARYNX. 


153 


Lymph-nodule 
of  pharyngeal 
tonsil 


Bundles  of 
muscular  tissue 
of  constrictors 


Stratified  squamous,  the  place  of  transition  being  subject  to  great  individual 
variation.  Where  covered  with  the  squamous  cells,  the  glands  are  usually 
mucous  in  tvpe.  The  lower  divisions  of  the  pharynx,  the  oro-  and  the 
laryngo-pharynx,  are  invested  with  stratified  squamous  epithelium. 

Lymphoid  tissue  occurs  in  great  abundance,  in  certain  localities, 
particularly  on  the  upper  posterior  pharyngeal  wall,  causing  the  mucous 
membrane  to  be  thrown  into  elevations.  The  larger  and  more  constant 
collections  of  lymphoid  tissue  are  called  ' '  tonsils, ' '  of  which  the  faucial 
tonsils  in  the  oro-pharynx,  the  pharyngeal  tp)isil  in  the  upper  part  of  the 
naso-pharynx,  the  tubal  tonsils 
at  the  openings  of  the  Eusta- 
chian tubes,  and  the  lingual 
tonsil  on  the  posterior  third  of 
the  tongue  are  examples. 

The  faucial  tonsils, 
also  called  palatine,  are  two 
almond-shaped  masses  of  lym- 
phoid tissue,  one  on  each  lat- 
eral wall  of  the  oro-pharynx, 
between  the  palatine  pillars. 
Each  tonsil,  some  20  mm. 
long,  15  mm.  broad  and  10 
mm.  thick,  is  enclosed  by 
a  fibro-elastic  capsule,  which 
becomes  continuous  with  the 
submucous  layer  where  the 
mucous  membrane  is  adherent 
to  the  tonsil,  as  it  is  on  the 
free  surface.  The  latter  is 
broken  by  an  uncertain  num- 
ber (10-20)  of  pits  of  varying 
size  and  depth.  These  depres- 
sions, or  crypts,  cut  the  lym- 
phoid tissue  into  irregular 
tracts,  which  are  still  further 
subdivided  by  connective  tis- 
sue septa  that  penetrate  from 
the  capsule.  The  crypts,  with 
their  side  branches,  are  com- 
pletely lined  with  mucous 
membrane  continued  from  the 
adjacent  free  surface.  The 
tunica  propria,  however,  is  thin 
and  so  invaded  by  the  lymphocytes  that  the  epithelium  is  the  most  conspicu- 
ous representative  of  the  mucous  membrane.  Each  crypt  with  its  surrounding 
tract  of  lymphoid  tissue  repeats,  in  its  general  makeup,  the  structure  of  the 
lingual  lymph-follicles  already  described.  The  lymphoid  mass  contains 
lymph-nodules,  with  germ-centres,  blended  together  by  the  more  diffuse 
lymphoid  tissue.  The  entire  tonsil  is  built  up  by  repetition  of  such  structural 
units.  Numbers  of  lymphocytes  wend  their  way  into  the  subepithelial  tissue, 
which  in  consequence  becomes  infiltrated  with  the  lymph-cells,  thence  pass 
into  the  epithelium  and,  finally,  escape  into  the  pit  and  onto  the  free  surface 
of  the  mucous  membrane.      These  cells  become  the  salivary  corpuscles. 


Fig.   192. — Sagittal  section  of  posterior  wall  of  pharynx  of 
child,  showing  part  of  pharyngeal  tonsil.     >   20. 


154 


NORMAL   HISTOLOGY. 


Muscular 
fibres 


The  pharyngeal  tonsil  is  an  unpaired  median  mass  of  lymphoid 
tissue  in  the  postero-superior  wall  of  the  naso-pharynx.  It,  as  well  as  the 
faucial  tonsils,  is  best  developed  in  early  childhood,  although  often  hyper- 

trophied  in  adolescence.  This  tonsil 
consists  of  a  series  of  lobulated 
masses  of  lymphoid  tissue  arranged 
around  a  central  depression  with 
lateral  recesses.  In  its  general 
composition,  it  resembles  the  faucial 
tonsil,  consisting  of  lymph-nodules 
and  diffuse  lymphoid  tissue.  The 
latter  is  less  circumscribed  and  infil- 
trates the  surrounding  mucous  mem- 
brane, so  that  the  limits  of  the 
pharyngeal  tonsil  are  not  well  defined. 
The  name,  tubal  tonsils,  is  some- 
times applied  to  the  collections  of 
lymphoid  tissue  that  surround  the 
openings  of  the  Eustachian  tubes. 
The  lymphoid  tract  extends  for  some 
distance  along  the  tube,  as  well  as 
towards  the  pharyngeal  tonsil,  and 
contains  small  lymph-nodules. 

The  blood-vessels  and  nerves 
of  the  pharynx,  although  collectively 
derived  from  a  varied  and  somewhat 
complex  source,  follow  in  their 
detailed  distribution  the  plan  com- 
mon to  mucous  membranes,  and  call 
for  no  special  description. 

The  lymphatics  within  the 
pharyngeal  mucous  membrane  are 
unusually  abundant,  particularly  in 
the  vicinity  of  the  more  definite  masses  of  lymphoid  tissue.  The  latter,  espe- 
cially the  faucial  tonsils,  are  of  great  practical  importance.  They  are  very 
frequently  the  seat  of  serious  infections,  their  numerous  and  often  deep  crypts 
affording  favorable  resting  places  for  bacteria. 


-Section  through  faucial  tonsil,  showing 
general  disposition  of  lymphoid  tissue  around  the 
crypts.    X  20. 


THE  CESOPHAGUS. 

The  oesophagus  or  gullet  is  the  tube,  about  25  cm.  in  length,  that 
connects  the  pharynx  and  the  stomach  and  serves  for  the  passage  of  food. 
Its  walls,  3-4  mm.  thick,  consist  of  four  coats  (Fig.  194)  which,  from  within 
outwards,  are:  (i)  the  mucous  membrane,  (2)  the  submucous  layer,  (3)  the 
muscular  tunic  and  (4)  the  fibrous  coat. 

The  mucous  membrane,  usually  thrown  into  longitudinal  folds, 
includes  a  tunica  propria,  composed  of  fibrous  connective  tissue  and  delicate 
elastic  fibres,  and  a  thick  coaling  of  stratified  squamous  epithelium.  The 
surface  of  the  stroma-layer  beneath  the  epithelium  is  modelled  by  longitudi- 
nal ridges  and  papillae,  between  which  pass  the  ducts  of  the  glands  in  their 
course  to  the  free  surface.  The  deeper  part  of  the  tunica  propria  is  occupied 
by  a  thin  stratum  of  involuntary  muscle,  the  muscularis  miicosce,  which  is 
feeble  and  indistinct  in  the  upper  part  of  the  gullet  but  robust  and  conspic- 


THE   (ESOPHAGUS. 


155 


uous  in  the  lower  portion  Collections  of  lymphoid  tissue  occur  within  the 
mucosa  as  more  or  less  distinct  aggregations.  Mostly  they  are  small  diffuse 
areas  around  the  ducts  of  the  mucous  glands,  but  in  some  places,  especially 
towards  the  lower  end  of  the  oesophagus,  they  assume  the  form  of  distinct 
lymph-nodules  (Fig.  195). 

The  submucous  layer,  although  of  considerable  thickness,  is  loose  in 
texture  and,  therefore,  permits  free  motion  of  the  mucous  membrane  upon 
the  muscular  tunic.      Scattered  along  the  length  of   the  gullet  are  many 

-Epithelium 


-Tunica  propria  of 
mucous  membrane 


# 


^luscularis 
mucosas 


'»" 


j^Gland-duct 


Glands 


Submucous  coat 


.     Circular 

-    iionstriated  muscle 


Striated  fibres 


^1 


-Longitudinal 
nonstriated  muscle 


Striate<1 

muscle-fibres 

Fig.  194. — Transverse  section  of  oesophagus,  through  upper  third.    X  18. 

CESophageal  glands.  These  are  of  two  kinds — the  ordinary  mucous 
glands,  situated  within  the  submucous  coat  and  most  abundant  in  the  upper 
half  of  the  tube,  and  the  special  cardiac  glands,  lodged  within  the  tunica 
propria  and  limited  to  the  two  ends  of  the  oesophagus.  The  usual  glands 
are  tubo-ah-eolar  in  form  and  pure  mucous  in  type,  mucus-producing  cells 
alone  being  present.  Their  ducts,  commonly  somewhat  tortuous,  are  often 
dilated  into  miniature  ampullae  just  before  penetrating  the  muscularis 
mucosae;  they  leave  the  tunica  propria  and  enter  the  epithelium  in  the  val- 
leys between  the  papillae.  The  smaller  ducts  are  lined  with  simple  columnar 
epithelium,  which  in  the  larger  tubes  may  become  stratified  and,  near  the 
free  surface,  may  be  replaced  by  stratified  squamous  cells. 

The  special  glands  correspond  in  structure  to  those  found  at  the  cardiac 
orifice  of  the  stomach  (page  1 60) ;  hence  ttheir  designation  as  "cardiac." 
They  consist  of  small  o\'al  or  pyramidal  groups  of  richly  branched  tubules 


156  NORMAL   HISTOLOGY. 

and  constitute  a  constant  narrow  zone  surrounding  the  termination  of  the 
oesophagus.  Lying  entirely  within  the  tunica  propria,  their  bases  abutting 
against  the  muscularis  mucosae,  they  open  on  the  free  surface  by  wavy 
ducts  that  pierce  the  summit  of  the  papillae  and  traverse  the  epithelium. 
Less  definite  and  much  less  constant  groups  of  similar  tubules  occur  within 
the  mucosa,  along  the  sides,  at  the  upper  end  of  the  oesophagus;  these  have 
been  described  as  the  superior  cardiac  gla7ids. 

The  muscular  coat  of  the  oesophagus  includes  an  inner  circular  and 
an  outer  longitudinal  layer,  although  the  individual  bundles  are  often  irregu- 
larly and  obliquely  disposed,  and  above  somewhat  intermingled.  The  histo- 
logical character  of  the  muscular  tissue  varies  in  different  parts  of  the  tube. 

Thus  in  a  general  way,  within 

.r:^:^,W;;,.i:;Uv^^:::?:~--..^  approximately  the  lower  half  of 

r;ris-&m^MM&^"  the  oesophagus  the    muscle  is 

<:Si^^^^^i^^J>"a*sivi,::.^  entirely  unstnped;  withm  the 

j^f^m^^^^K^IIM0Sii/^^^0;: :       second   quarter    both    striated 

Lymph-noduiei^^^El^Htt§iS|i|^  and  unstriped  muscle  appear; 

in  mucosa^  ^^^'t<^^^M^M§B-  't  ^^-■- ■-?^".       while  within  the  first  quarter 

■f'^^y^d ''^■:\-'jr/ '''''■' ^'^'P'f:W':;-' ^''-  Striated    muscle  almost  exclu- 

C  .  sively    is    present.     Although 

5?£?  "  -      within  the   longitudinal   layer 

^;^^r  .,  unstriped  fibres  do  not  appear 

SL^r  within  the  upper  fourth  or  fifth. 

Submucous  layer  Ml^„:___:              ^        >  ^^    zone    of    purely   Striated 

:^^;  fibres  withm  the  circular  layer 

%|J^  includes  only  about   the    first 

f^-    '     /  2.5  cm. ,  below  which  level  un- 

Muscie  JiSll—^-Jbi— : V     ■  striped  fibres  gradually  become 

-^  ■^:A:y.  ^l:!-'  ' -,1:^.     ,  -:;       more  abundant. 

"  The  fibrous  coat  is  poor- 

FiG.  igs. — Section  of  mucous  membrane  of  cesophaffus,      1        j         1  j       i_  >.i         j- 

showing  lymph-nodule.  X  55.  "^  *  ty  developed  abovc  the  dia- 
phragm, consisting  of  areolar 
tissue  that  attaches  the  oesophagus  to  the  surrounding  structures,  aided  in 
places  by  strands  of  unstriped  muscle.  After  piercing  the  diaphragm,  the 
peritoneum  contributes  a  limited  serous  coat,  which,  with  its  connective 
tissue  stroma  and  surface  mesothelium,  constitutes  a  more  definite  investment 
for  the  alimentary  tube  from  this  point  on. 

The  blood-vessels  of  the  oesophagus,  after  sending  twigs  directly  for 
the  supply  of  the  muscular  tissue,  gain  the  loose  submucous  layer,  within 
which  lie  branches  of  considerable  size.  From  these,  in  addition  to  small 
vessels  for  the  circular  muscle  and  the  glands  within  the  submucosa,  twigs 
are  given  off  that  enter  the  mucous  membrane  and  break  up  into  capillary 
networks  from  which  terminal  loops  invade  the  papillae.  The  venous  radi- 
cles of  the  mucosa  are  tributary  to  the  veins  within  the  submucous  tissue, 
from  which  the  larger  trunks  accompany  the  arteries. 

The  lymphatics  are  represented  by  networks  within  the  submucous 
and  muscular  coats.  The  former  take  up  the  fluid  collected  by  the  lymph- 
spaces  of  the  mucous  membrane.  These  networks  are  connected  by  inde- 
pendent trunks  with  neighboring  lymph-nodes,  those  from  the  lower  end 
of  the  oesophagus  passing  to  the  upper  nodes  of  the  coeliac  group. 

The  nerves  of  the  oesophagus  include  both  medullated  and  nonmedul- 
lated  fibres.  The  former,  largely  from  the  vagi,  contribute  fibres  supplying 
the  striated  muscle,  in  which    they  end    in    motor   plates.     The    nonnied-  ' 


THE   STOMACH.  157 

ullated  fibres  are  chiefly  sympathetic  and,  therefore,  are  destined  espe- 
cially for  the  involuntary  muscle  and  glands.  Between  the  longitudinal  and 
circular  layers  of  muscle  they  form  a  wide-meshed  plexus,  containing  many 
microscopic  ganglia,  that  corresponds  with  the  plexus  myenteriais  of  the 
stomach  and  intestine,  with  which  organs  it  will  be  described  (page  163).  The 
mucous  membrane  receives  nonmedullated  fibres,  from  an  indefinite  plexus 
within  the  submucous  layer,  which  terminate  within  the  tunica  propria  in 
free  endings,  some  threads  entering  the  deeper  layers  of  the  epithelium. 

THE   STOMACH. 

Being  essentially  a  greatly  dilated  and  modified  segment  of  the  digestive 
tube,  connecting  the  oesophagus  and  the  small  intestine,  the  walls  of  the 
stomach  agree  in  their  general  makeup  with  those  of  the  other  parts  of  the 


■'fr 

Gastric  glands 

Mucosa  "^    •'     ^r^  ^  Muscularis 

mucosae 


"*^~,,     ^>~.  ^ Blood-vessel 


Subniucosa 


Oblique  bundles 
of  iionstriated 
muscle 


Serosa 
Fig.  196. — Transverse  section  of  human  stomach,  left  third,  showing  general  arrangement  of  coats.    X  i8. 

alimentary  canal  lying  below  the  diaphragm.  They  consist,  therefore,  of 
four  coats— the  mucous,  the  submucous,  the  muscular,  and  the  serous. 
When  moderately  distended,  the  stomach  is  a  somewhat  flattened  pear- 
shaped  sac,  with  the  large  end  up  and  the  point  bent  to  the  right.  The 
highest  part  of  the  stomach  is  called  Xkx^  fundus  ;  the  end  joining  the  oesoph- 
agus is  the  cardia,  and  that  meeting  the  intestine  the  pylorus. 

The  mucous  coat,  or  mucosa,  thickest  near  the  pylorus  (1.5-2  mm.) 
and  thinnest  at  the  cardiac  end  (.35-55  mm.),  is  loosely  attached  to  the 
muscle  by  the  submucous  tissue  and,  possessing  little  elasticity,  is  easily 
thrown  into  folds  or  rugcB  when  the  other  coats  contract.  During  disten- 
tion these  folds  are  largely  effaced,  but  under  the  more  usual  conditions  are 


158 


NORIMAL    HISTOLOGY. 


S'-rAa 


conspicuous  as  longitudinal  plications,  particular!}'  at  the  pyloric  end  and 
along  the  greater  curvature.  The  surface  of  the  gastric  mucous  membrane 
is  divided  into  small,  slightly  raised,  polygonal  areas  or  inainmillce.  The 
latter,  from  1—4  mm.  in  diameter,  are  stippled  with  closely  placed  micro- 
scopic pits,  the  gastric  C7ypts,  into  which  open  groups  of  the  individual 
gland-tubes.  The  crv^pts  are  particularly  wide  (.2  mm.)  in  the  pyloric 
region,  where  they  reduce  the  intervening  tissue  to  mere  ridges  which  in 
section  appear  as  villous  projections  (^pliccB  villosa^. 
The  epitheluim  covering  the  free  surface  of  the 
mucosa  is  a  single  layer  of  tall  columnar  cells  (20— 30,^. 
high),  many  of  which  are  engaged  in  secreting  mucus 
and  consequently  appear  as  goblet-cells.  Where  the 
oesophagus  joins  the  stomach,  the  opaque  stratified 
squamous  epithelium  of  the  gullet  abruptly  changes 
into  the  transparent  columnar  gastric  cells.  'The  line 
of  transition  is  zigzag  and  well  defined,  the  pale  oesoph- 
ageal mucous  membrane  contrasting  with  the  reddish- 
gray  gastric  lining.  The  unusual  thickness  of  the  mu- 
cosa at  the  pylorus  is  in  part  due  to  the  depth  of  the  de- 
pressions, the  gastric  crypts,  into  which  the  glands  open. 
The  tunica  propria  consists  of  a  delicate  con- 
nective tissue  framework,  composed  of  fibrous  and 
reticular  tissue  and  elastic  fibres,  supporting  many 
lymphocytes  and  intermingled  with  strands  of  invol- 
untary muscle  and  capillaries.  This  stroma  resembles 
loose  lymphoid  tissue  and,  where  the  gastric  glands 
are  closely  placed,  is  reduced  to  the  septa  and  envel- 
opes which  separate  and  invest  the  deeper  parts  of  the 
gastric  glands.  The  latter,  here  and  ther^,  are  so 
surrounded  by  aggregations  of  lymphocytes  that  the 
stroma  assumes  the  appearance  of  lymphoid  tissue. 
In  the  vicinity  of  the  pylorus  and  sometimes  of  the 
^cardia,  more  definite  accumulations  of  such  tissue  occur 
\in  the  form  of  small  lymph-nodules  (Fig.  200),  the 
so-called  "lenticular  glands."  Occasionally  they  are 
of  sufhcient  size  to  reach  almost  the  free  surface, 
although  commonly  they  are  limited  to  the  deeper  parts 
of  the  mucous  membrane.  The  musculai-is  mucosae,  as 
in  other  parts  of  the  intestinal  tube,  consists  of  a  thin 
but  well-marked  layer  of  involuntary  muscle  within 
the  tunica  propria  next  the  submucous  coat.  Two 
strata  are  usually  distinguishable,  an  inner  circular 
and  an  outer  longitudinal.  Delicate  strands  of  muscle- 
cells  extend  between  the  gastric  glands,  in  places  penetrating  almost  as  far 
as  the  epithelium. 

The  gastric  glands  comprise  two  principal  varieties,  the  fitndus  and 
the  pyloric  glands.  The  former  occur  throughout  the  greater  part  of  the 
stomach,  including  the  fundus,  the  anterior  and  the  posterior  walls  and  the 
curvatures;  the  latter  are  present  in  the  pyloric  fifth  of  the  stomach.  An 
additional  variety,  the  cardiac  glands,  is  represented  by  a  narrow  granular 
group  at  the  oesophageal  orifice. 

The  fundus  glands — the  gastric  glands  proper — are  closely  set  tubules, 
single  or  slightly  branched  and  usually  somewhat  waAy,  which  extend  almost 


V-i^ 


Fig.  197. — Gastric  glands 
from  fundus-end  of  stom- 
ach ;  a,  opening  on  sur- 
face; b,  neck;  c,  fundus; 
rf,  parietal  cells ;  e,  chief 
cells.     X  105. 


THE   STOMACH. 


159 


/I 


(r 


Sccretion- 

LJiialiculus 


Interglandular 
tissue 


the  entire  thickness  of  the  mucosa  and  abut  against  the  muscularis  mucosae. 
Each  gastric  crypt  corresponds  to  an  excretory  duct  and  receives  se\eral  of 
the  tubules,  in  wliich  the  neck,  body  and  fundus  of  the  gland  are  recognized. 
The  slightly  constricted  neck  marks  the  transition  of  the  columnar  epithelium 
lining  the  crypt,  prolonged  from  the  free  surface,  into  the  gland-cells.  The 
latter  are  of  two  kinds,  the  chief  and  the  parietal.  The  chief  cells,  called  also 
central  or  adelomorphous,  are  clear  low  cylindrical  or  cuboidal  elements  that 
line  the  tubule  and  surround  a  narrow  but  distinct  lumen.  Their  spherical 
nuclei  occupy  the  central  part  of  the 
clear  cytoplasm  that  sometimes  (dur- 
ing rest)  is  filled  with  secretion  or 
zymogen  granules  and  at  other  times, 
after  the  granules  have  disappeared, 
shows  a  distinct  reticulum.  These 
elements,  often  imperfectly  preserved, 
are  probably  concerned  in  producing 
pepsin.  The  parietal  cells,  known 
also  as  acid,  oxyntic  or  delomor- 
phous,  although  relatively  few  are 
conspicuous  by  reason  of  their  size, 
peripheral  position  (Fig.  198)  and 
selective  affinity  for  certain  stains. 
They  are  large,  of  rounded  triangular 
form,  and  lie  immediately  beneath  the 
basement  membrane,  which  often 
displays  in  profile  an  outward  bulg- 
ing as  it  passes  over  the  cell.  The 
finely  granular  cytoplasm  encloses  a 
distinct  spherical  nucleus  that  may 
be  double.  Although  arranged  with 
little  regularity,  the  parietal  cells  are 
most  numerous  towards  the  neck  of 
the  tubule,  where  they  may  form  an 
almost  unbroken  layer;  they  decrease 
in  number  on  approaching  the  fundus, 
in  which  they  are  few  or  wanting. 
While  excluded  by  the  chief  cells 
from  contact  with  the  lumen  of  the 
tubule,  the  parietal  cells  are  connected 
with  the  central  cleft  by  means  of 
lateral  channels,  the  secretion-eanaliculi,  that  pass  outwards  between  the 
chief  cells  and  enclose  the  parietal  cells  with  networks. 

The  pyloric  glands  differ  from  the  fundus  glands  in  the  excessive  width 
and  depth  of  their  crypts,  the  excretory  ducts,  into  which  groups  of  relatively 
short  tortuous  gland-tubules  open,  and  in  the  character  of  their  lining  cells. 
The  latter  are  almost  exclusively  columnar  or  pyramidal  elements  which 
resemble  the  chief  cells  of  the  fundus  glands.  That,  however,  they  are  at 
least  functionally  different  is  shown  by  their  beha\'ior  with  certain  stains,  as 
well  as  by  the  mucus  reaction  of  their  secretion.  The  spherical  nuclei  com- 
monly lie  near  the  basement  membrane.  Occasional  parietal  cells  are  also 
present.  Owing  to  their  tortuous  course,  the  deeper  parts  of  the  pyloric 
tubules,  when  viewed  in  sections,  are  cut  in  various  planes.  Within  the 
intermediate  zone  between  the  pyloric  and  adjoining  portions  of  the  stomach. 


I. 


r 


Lumen  of 
gland 


-Chief  cell 


-Parietal  cell 


n 


r 


Fig.  198 — Deep  portions  of  gastric  glands  from 
fundus,  showing  the  two  varieties  of  lining  cells 
and  secretion-canaliculi  connecting  parietal  cells 
with  the  gland-luniina.     X  315. 


i6o  NORMAL   HISTOLOGY. 

the  pyloric  and  the  fundus  glands  are  intermingled.  Passing  towards  the 
intestine,  the  transition  of  the  pyloric  glands  into  those  of  the  duodenum  is 
gradual,  the  gastric  tubules  sinking  through  the  muscularis  mucosae  until, 
as  the  duodenal  glands,  they  occupy  the  submucosa. 

The  cardiac  glands  form  a  narrow  annular  group,  some  5  mm.  in 
width,  surrounding  the  orifice  of  the  oesophagus.  According  to  the  char- 
acter of  their  cells,   which  resemble  the  chief  cells  of  the  gastric  tubules, 

xpMn  ^ 

^k  Id "  ^ 


•.''V/    /_ 


^^■^  ^t^, 


Orifice  of  glands 


-^^                             r  V     f r,                                                                    /Mucous  coat 
"^f^  '-"  '  "^  ^^^"i^^  ''-""'  =  '  '     '  Pyloric  glands 


_Muscularis 
mucosse 


"  Submucous  coat 


■■-^a 


-Circular  muscle 


.Longitudinal 
muscle   ' 


-Serous  coat 


Fig.  199.— Transverse  section  of  human  stomach,  pyloric  end  ;  a  ruga  is  cut  across,  showing  the  mucosa 
supported  by  core  of  submucous  tissue.     X  18. 

with  a  few  parietal  cells,  the  cardiac  glands  may  be  regarded  as  modified 
fundus  tubules;  their  mucus  reactions  and  repeatedly  branched  condition, 
however,  suggest  similarity  to  the  pyloric  glands.  Their  excretory  crypts 
often  exhibit  ampuUary  enlargements.  Among  the  more  distinctive  cardiac 
tubules  are  usually  a  few  shorter  ones  which  recall  the  intestinal  crypts  of 
Lieberklihn. 

The  submucous  coat  consists  of  loose  fibro-elastic  connective  tissue 
and  allows  the  mucous  membrane  to  move  freely  on  the  muscular  coat.  In 
the  more  permanent  rugae,  the  submucous  tissue  forms  the  core  of  the 
elevation.  The  submucosa  contains  blood-vessels  of  considerable  size,  a 
meshwork  of  lymphatics  and  the  nerve-plexus  of  Meissner,  as  well  as 
occasional  groups  of  fat-cells. 


THE   STOMACH. 


i6i 


i 


mi. 


j^y 


~4 


Mouth  of 
gland  — 


^v- 


Pj'loric 
gland 


The  muscular  coat,  composed  entirely  of  unstriped  muscle,  comprises 
three  irregiUar  layers — the  external  of  longitudinal,  the  middle  of  circular 
and  the  inner  of  oblique  fibres.  None  of  these  layers  is  complete  in  all 
parts  of  the  stomach,  the  circular  being  the  least  imperfect  and  usually  the 
most  conspicuous.  The  external  layer  consists  of  longitudinal  fibres,  con- 
tinuous with  the  corresponding  ones  of  the  oesophagus  above  and  with  those 
of  the  duodenum  below.  It  is  best  developed  along  the  lesser  curvature, 
over  the  greater  curvature  and  intervening  surfaces  being  a  thin  and  irregular 
stratum.  Towards  the  pylorus  the  longitudinal  fibres  form  a  more  compact 
and  thicker  layer,  which  passes  without  interruption  over  the  gastro-intestinal 
'junction  into  the  outer  mus- 
cle of  the  duodenum.  The 
middle   layer,   while    com-  "^ 

plete  and  circularly  dis- 
posed within  the  pyloric 
end,  is  imperfect  and  ob- 
lique towards  the  fundus. 
It  is  continuous  above  with 
the  superficial  circular  fibres 
of  the  gullet;  these,  on 
reaching  the  stomach,  how- 
ever, are  arranged  as  loops, 
which  overlie  the  lesser 
curvature  but  do  not  reach 
the  fundus.  As  these  loops 
pass  downward  they  in- 
crease in  length  and  regu- 
larity until,  at  the  middle  of 
the  stomach,  the  circular 
strands  completely  girdle 
the  organ.  Towards  the 
pylorus  the  circular  bundles 
thicken  and,  at  the  imme- 
diate end  of  the  stomach, 
surround  the  opening  into 
the  intestine  with  a  robust 
muscular  ring,  the  pyloric 
splmicter,  the  outer  part  of 
which  alone  continues  into 
the  duodenum.  The  inner 
layer,  the  least  complete  and  most  oblique,  begins  at  the  left  of  the  oesopha- 
geal orifice  as  a  prolongation  of  the  deeper  circular  fibres  of  the  gullet. 
On  the  stomach  the  fibres  are  disposed  as  loops  which  cover  the  fundus  with 
a  fairly  continuous  layer,  but  become  progressively  more  oblique  and  incom- 
plete over  the  surfaces  of  the  middle  third,  being  absent  o\'er  the  lesser 
curvature  and  the  pyloric  fourth  of  the  stomach.  It  is  evident,  therefore, 
that  within  the  narrow  tubular  part  of  the  organ,  the  muscle  layers 
have  the  most  definite  and  orderly  disposition;  here  they  show  in 
cross-sections  as  a  well-developed  inner  circular  and  a  definite  external 
longitudinal  stratum  (Fig.  199). 

The  serous  coat  corresponds  in  structure  with  other  portions  of  the 
visceral  peritoneum,   consisting  of  the  surface  mesothelial  plates  and   the 
subjacent  tunica  propria  of  fibro-elastic  connective  tissue. 
II 


Y 

% 


Fundus  of 
gland  — ^. 


Lymph- 
nodule' 


Muscularis 
mucosce- 


FiG.  2c». — Section  of  pyloric  end  of  stomach,  showing  glands  and 
part  of  a  lymph-nodule.    X  182. 


l62 


NORMAL   HISTOLOGY. 


The  blood-vessels  of  the  stomach  inckide  arterial  branches  from  the 
three  subdivisions  of  the  coeliac  axis.  At  first  the  arteries  lie  just  beneath 
the  peritoneum,  between  the  folds  of  which  they  gain  the  stomach  and  to 
which  they  give  off  branches.  They  then  pierce  the  muscular  tunic,  whose 
outer  part  is  supplied  during  their  passage.  On  reaching  the  submucous 
coat,  the  arteries,  still  of  considerable  size,  form  a  coarse  network,  from 
which  some  small  vessels  pass  into  the  muscular  layer  to  complete  its  supply, 
while  many  more  penetrate  the  muscularis  mucosae  and  enter  the  tunica 
propria.  Here  they  form  a  network  beneath  the  glands  from  which  pass 
slender  capillaries  to  surround  the  gastric  tubules  and  to  encircle  the  mouths 


Mucosa 


Submucosa  =^lf#-;i2i 


Muscularis  •Seiffl 


Serosa  '=- 


Fig.  201. — Transverse  section  of  injected  stoniacii.     X  50. 


of  the  glands  beneath  the  epithelium.  From  these  superficial  capillaries  the 
blood  is  returned  by  relatively  straight  unbranching  veins  that  pass  betw^een 
the  gland-tubules  and  join  the  venous  plexus  in  the  deepest  part  of  the 
tunica  propria.  Thence  tributaries  join  the  submucous  venous  plexus  from 
which  the  larger  trunks  accompany  the  arteries.  Although  the  important 
tributaries  of  the  portal  system  are  devoid  of  valves,  the  veins  which  imme- 
diately drain  the  stomach  are  provided  with  such  folds. 

The  lymphatics,  or  chyle  vessels,  begin  within  the  tunica  propria  as 
blind  capillaries  that  course  between  the  gland-tubules  as  far  as  the  close 
network  in  the  depth  of  the  stroma.  Numerous  channels  establish  com- 
munication between  the  lymph-paths  of  the  mucosa  and  the  wide-meshed 
plexus  of  larger  vessels  within  the  submucous  coat.  A  second  network  of 
lymph-capillaries  extends  between  the  layers  of  muscle  and  joins  the  efferent 
lymph-paths  that  connect  the  gastric  walls  with  the  neighboring  nodes. 


THE   SMALL    LNTESTINE.  163 

The  nerves  of  the  stomach  are  from  the  vagi  and  the  sympathetic  and 
contain  both  meduUated  and  nonmedullated  fibres,  the  latter  {predominating. 
After  forming  a  subserous  plexus  beneath  the  peritoneum,  thev  pierce  the 
external    layer   of    longituciinal 
muscle  and  between  the  latter 
and  the  circular  muscle  broaden 

out   and   unite   into   X[\q  plexus  ^  -'-^-V:--  '•'^■.  ^-,. ^ 

myentericus  or  plexus  of  Auer-     "^S^^  '  ^^S^T' 


dacli.       This    is     an    extended 

network,  with  rounded  angular  ■%  ..-i^^.^  -""^  1 


meshes,  whose  points  of  inter- 
section are  occupied  by  micro- 
scopic ganglia  composed  of  small 
groups  of  sympathetic  cells. 
From     these    numerous    fibres         -"    '  .     "^ 

pass    to  the    adjoining    sheets    of         Fig.   202.— surface  view  of   muscular  coat  of  stomach, 

d.  1      4.      i  •       i        sliowino- groups  of  eanglion-cells  and  nerve-fibres  of  plexus 

untary  muscle  to  termmate    of  Auerbach.   x  50 

in  free  endings  among  the  fibre- 
cells  (page  86).  Other  fibres  form  the  intramuscular  plexus  and  pene- 
trate, as  obliquely  directed  bundles,  the  intervening  muscle  to  gain  the 
submucous  coat.  Within  the  latter  they  form  the  submucous  plexus  or 
plexus  of  Meissner,  which,  while  resembling  the  intramuscular  network 
in  its  general  features,  is  less  pronounced,  finer  meshed  and  beset  with 
smaller  ganglia.  Numerous  nonmedullated  fibres  leave  the  submucous 
plexus  to  enter  the  overlving  tunica  propria,  in  which  some  end  in  delicate 
plexiform  threads  around  the  gastric  glands  and  others  in  fibrils  for  the 
muscularis  mucosae.  MeduUated  fibres,  dendrites  of  sensory  neurones,  are 
also  present  within  the  mucosa,  where  they  form  a  subepithelial  plexus  after 
losing  their  medullary  coat.  They  end  in  minute  varicose  threads  within 
the  tunica  propria;  whether  some  fibrils  pass  between  the  epithelial  cells  is 
uncertain. 

THE   SMALL    INTESTINE. 

The  small  intestine,  about  7  meters  or  23  ft.  in  length,  is  convention- 
ally dix'ided  into  three  parts — the  duodenum,  the  jejunum  and  the  ileum. 
Although  typical  portions  of  these  segments  can  be  readily  distinguished 
from  one  another,  chiefly  by  the  modifications  of  the  mucous  coat,  the 
transition  between  them  is  so  gradual  that  differentiation  is  in  places  impos- 
sible. The  small  intestine,  as  other  parts  of  the  alimentary  tube  below  the 
diaphragm,  consists  of  four  coats— the  mucous,  the  submucous,  the  muscular 
and  the  serous. 

The  mucous  membrane,  or  mucosa,  presents  the  greatest  variations, 
since  its  function  as  an  absorbent  surface  requires  an  extent  of  area  most 
economically  provided  by  folds  and  projections.  These  elevations  of  the 
mucosa,  which  include  plications  and  villi,  are  most  marked  in  the  upper 
part  of  the  intestine,  where  absorption  is  most  active,  thence  gradually 
decreasing  until  in  the  terminal  part,  where  the  small  intestine  passes  into 
the  large,  they  almost  disappear. 

The  epithelium,  everywhere  covering  the  free  surface,  including  the  villi, 
consists  of  a  single  layer  of  columnar  cells,  whose  ends  next  the  intestinal 
lumen  are  invested  by  a  narrow  and  often  delicately  striated  cuticular  border. 
The  latter,  present  only  in  the  fully  matured  cells,  lacks  stability  and  is  readily 
resolved  into  minute  vertical  rods,   probably  continuous  with  the  spongio- 


164 


NORMAL   HISTOLOGY. 


plastic  threads  within  the  body  of  the  cell.  In  places,  especially  over  the 
villi,  many  of  the  epithelial  elements  are  engaged  in  producing  mucus  and, 
hence,  appear  as  goblet-cells.  Migratory  lymphocytes  are  usually  to  be 
seen  between  the  cells,  while  during  digestion  the  cytoplasm  of  the  latter  is 
often  loaded  with  particles  of  fat.  The  oval  nuclei  occupy  the  deeper  parts 
of  the  cells.  The  tunica  propria  of  the  intestinal  mucosa  resembles  lymphoid 
tissue,  since  it  consists  of  a  delicate  connective  tissue  reticulum  containing 
numerous  small  round  cells  similar  to  lymphocytes.      This  stroma  fills  the 


Villui 


Duct  of  Brunner's 

glands 

Muscularis  mucosse^- 


Crypt  of  Lieberkiihn 


^7 
^^1 


1^ 


Brunner's  glands  ^ 


'T:.... 


Serous  coat ■ 


Brunner's  glands 


Circular  muscle 
Longitudinal  muscle 


Fig.  203.— Transverse  section  of  small  intestine  (duodenum'),  showing  general  arrangement  of  coats  and 
the  two  varieties  of  glands.    X  90. 

Spaces  between  the  glands  and  forms  the  core  of  the  villi  over  which  the 
epithelium  stretches.  The  deeper  part  of  the  mucosa  is  occupied  by  a  well- 
marked  muscularis  mucoscs,  composed  of  an  inner  circular  and  an  outer 
longitudinal  layer  of  unstriped  muscle. 

The  villi  are  minute  projections  of  the  mucosa,  barely  visible  to  the 
unaided  eye,  whose  presence  imparts  the  characteristic  velvety  appearance  to 
the  fnner  surface  of  the  small  intestine.  Although  found  throughout  the 
latter  (but  absent  in  the  large  gut),  they  are  most  abundant  (20-40  to  the 
sq.  mm.)  in  the  duodenum  and  the  jejunum  and  less  numerous  (15-30  to 
the  sq.    mm. )  in  the  ileum.      In  the  duodenum  they  appear  close  to  the 


THE   SMALL    INTESTINE. 


165 


pylorus,  but  are  better  developed  in  the  second  part,  where  they  are  low  and 
broad  and  measure  .2-5  mm.  in  height  and  .3-1  mm.  in  width.  In  the 
jejunum  the  villi  are  conical  and  somewhat  laterally  compressed,  while  in 
the  ileum  their  shape  is  cylindrical,  filiform  or  wedge-like  and  their  height 
from  .5-1  mm.  The  villi  are  projections  of  the  mucous  coat  alone  (Fig. 
204)  and  consist  of  a  framework  of  the  lymphoid  stroma-tissue,  covered  bv 
columnar  epithi^lium,  supporting  the  absorbent  vessel  and  the  blood-vessels 


Stroma  of  tunica  propria 


Lacteal 


Surface  epithelium 


Goblet-cell 


<Tland  of  Lieberkiilm 


Muscularis  mucosa 


Submucous  coat 


Circular  muscle £:^Mimjh 


Fig.  204. — Transverse  section  of  small  intestine  (jejunum),  showing  the  villi  cut  lengthwise. 


and  intermingled  with  a  few  strands  of  unstriped  muscle.  The  supporting 
framework  of  the  villus — a  complex  of  fibrous,  reticular  and  elastic  tissue — is 
condensed  beneath  the  epithelium  into  a  delicate  membrane.  The  lacteal, 
as  the  absorbent  vessel  or  lymphatic  occupying  the  villus  is  usually  termed, 
begins  as  a  blind,  often  slightly  club-shaped  channel,  which  runs  through 
the  centre  of  the  villus,  surrounded  by  the  delicate  muscle-bundles  and  the 
blood-capillaries.  While  the  slender  cylindrical  villi  possess  a  single  lacteal 
(25—35  !'■  ii''  diameter),  those  of  broader  form  often  contain  two,  three  or  even 
more  such  \'essels,  which  may  communicate  by  cross-channels.      Their  walls 


1 66 


NORMAL    HISTOLOGY. 


consist  of  a  single  layer  of  endothelial  plates  and  are  surrounded  by  the 
strands  of  muscle.  While  the  absorbent  vessels  within  the  villi  are  at  times 
conspicuous  by  reason  of  the  particles  of  fat  which  they  contain,  and  hence 

are  called  "lacteals,"  they 

^         '^l^ Gohlet-cell 


^r^*^~^    ,,«mt5gis4*r-i^S^  ''     f'v  Cuticular 


Capillary 

Cuticular  border 
um 


Lacteal 


Fig.  205. — Transverse  section  of  a  single  villus,  showing  relation 
of  epithelium,  stroma  and  vessels.     X  350. 


only  blind  lymph- 
radicles  and  actually  be- 
long to  the  system  of 
lymphatics.  Their  special 
purpose  is  to  carry  the 
materials  taken  from  the 
intestinal  contents  to  the 
great  lymph-channel,  the 
thoracic  duct. 

The  plicae  circula- 
res,  or  valvidce  conniven- 
tes,  within  the  duodenum 
and  jejunum,  additionally 
model  the  mucous  mem- 
brane and  greatly  increase 
its  secreting  and  absorbing 
surface,  as  well  as  retard 
the  passage  of  the  intes- 
tinal contents,  thereby 
facilitating  the  digesti^'e 
processes.  These  transverse  folds  begin  in  the  second  part  of  the  duo- 
denum and  are  duplicatures  which  involve  not  only  the  entire  thickness 
of  the  mucosa,  but  contain  a  central  supporting  projection  of  the  submucous 
coat  (Fig.  207);  hence  they  can  not,  as 
a  rule,  be  effaced  by  distention.  The 
height  of  the  folds,  where  well  developed, 
rarely  exceeds  6  mm. ,  and  towards  the  lower 
part  of  the  jejunum  is  much  less;  in  the 
terminal  portion  of  the  ileum  they  usually 
are  wanting. 

Glands. — The  structures  within  the  wall 
of  the  intestinal  tube  to  which  the  term 
"glands"  has  been  applied  include  two 
entirely  different  groups — the  true  secreting 
organs,  the  glands  of  Brunner  and  the 
crypts  of  Lieberkiihn,  and  the  accumula- 
tions of  lymphoid  tissue,  the  single  or 
aggregated  lymph-nodules,  which  do  not 
secrete. 

The  glands  of  Brunner  or  duodenal 
glands  (  P'ig.  208)  are  limited  to  the  first 
division  of  the  small  intestine.  Beginning  at 
the  pylorus,  where  they  are  most  numerous 
and  extensive,  they  gradually  decrease  in 
number  and  size,  until,  at  the  lower  end  of  the  duodenum,  they  are  entirely 
wanting.  In  the  vicinity  of  the  opening  of  the  bile-duct,  however,  they  are 
locally  augmented.  These  glands  are  direct  continuations  of  the  pyloric 
glands  of  the  stomach,  with  which  they  agree  in  all  essential  structural 
details.      They  are   not   confined    to   the    mucous   coat,  as  are    the  gastric 


Fig  2c6 — Suitace  Mev\  ot  mucous 
memorane  of  jejunum  ,  buippicd  appear- 
ance is  due  to  villi,  which  cover  also  the 
folds.     Natural  size. 


THE   SMALL    INTESTINE. 


167 


glands,  but  occupy  chiefly  the  sul^mucosa  (Fig-.  209).      The  upper  part  of 
the    cluodenuni    possesses,    therefore,    a    double   layer  of   true   glands — the 


.Mucosa 

^  Sub- 
-^''^      mucosa 


ir/^' 


) 


Circular 
muscle 


Fig.  207. —  Longitudinal  section  of  duodenum  ;  plicae  circulares  are  cut  across,  showing  relation  of  these 

folds  to  the  villi.      ■    ii. 

crypts  of  Lieberkiihn  within  the  mucous  coat,  beneath  which,  in  the  sub- 
mucosa,  lie  the  glands  of  Brunner.      The  individual  duodenal  glands,  tubo- 


Duct  of  Brunqer's  gland        Villus 


Gland  of  Lieberkiihn 


Lymph-' — 
nodule      ' 


Brunner's^ 
glands 


^m 


Circular- 
muscle 


Longitudinal- 
muscle 
Serous  coat' 


Fig.  20S. — Longitudinal  section  of  duodenum,  showing   Brunner's  and  Lieberkiihn's  glands,  villi  and 

lymph-nodule.     X  68. 

alveolar  mucous  in  type,  form  somewhat  flattened  spherical  or  polygonal 
masses  (.5-2  mm.),  consisting  of  richly  branched  tubules  ending  in  dila- 
tations.    Their  excretory  ducts    pierce  the   muscularis  mucosae  and,    after 


1 68 


NORMAL    HISTOLOGY. 


traversing  the  tunica  propria,  open  either  directly  on  the  free  surface,  or  into 
the  crypts  of  Lieberkiihn.  The  columnar  gland-cells  lining  the  duodenal 
alveoli  are  probably  identical  in  nature  with  those  of  the  pyloric  glands. 

The  crypts  or  glands  of  Lieberkiihn  are  simple  tubular  depressions 
which  are  found  not  only  throughout  the  small  intestine,  but  the  large  one 
as  well.  Under  low  magnification,  the  surface  of  the  small  intestine  exhibits 
numerous  pits,  the  orifices  of  these  crypts,  which  almost  fill  the  spaces 
between  the  bases  of  the  villi;  with  the  exception  of  the  areas  immediately 
overlying  the  lymph-nodules,  where  they  are  displaced,  these  glands  are 
present  in  all  parts  of  the  intestinal  tube.  They  are  very  closely  set,  narrow 
and  penetrate  the  tunica  propria  as  far  as  the  muscularis  mucosae.  In  length 
they  vary  from  .2-4  rnm.,  and  in  diameter  from  60-80  //.  The  lining  of  the 
crypts  rests  upon  a  delicate  basement  membrane  and  consists  of  a  single 


Pyloric  glands 


Mucosa 


Submucosa 


Longitudinal 
muscle 


Duodenum 


Fig.  209.— Longitudinal  section  through  junction   of  stomach  and  duodenum,  showing  transition  of 
pyloric  into  duodenal  glands ;  also  thickening  of  circular  muscle  to  form  sphincter  pylori..    X  15. 


layer  of  columnar  cells,  continuous  with  those  covering  the  villi.  They  differ 
from  these  in  being  shorter  and  without  the  cuticular  border  seen  on  the 
villi.  In  view  of  the  presence  of  mitotic  figures  within  the  crypt-epithelium 
of  the  adult,  it  is  probable  that  the  young  cells  here  produced  are  gradually 
pushed  upwards  and  supply  the  elements  required  to  replace  the  old  worn- 
out  surface  cells  on  the  villi.  Goblet-cells,  as  well  as  migratory  colorless 
blood-corpuscles,  occur  among  the  lining  of  the  crypts.  Constantly  within 
the  human  ileum,  the  deepest  parts  of  the  crypts  of  Lieberkiihn  contain 
small  groups  of  granular  elements,  the  ce//s  of  Paneth,  whose  significance 
is  undetermined.  The  duodenum,  jejunum  and  vermiform  appendix  are 
uncertain  seats  of  these  cells,  while  within  the  large  intestine  they  are  absent. 
The  fact  that  such  cells  are  wanting  in  many  orders  of  animals  possessing 
well  developed  intestinal  crypts,  points  to  some  special,  rather  than  a  general, 
purpose. 

Lymph-Nodules. — The  lymphoid  tissue  within  the  intestinal  tube 
occurs  in  the  form  of  circumscribed  nodules,  which  may  remain  isolated,  as  the 
solitary  nodules,  or  be  collected  into  considerable  masses,  as  Peyer  s  patches. 

The  solitary  nodules  vary  greatly  in  number  and  size,  sometimes 
being  abundant  in  all  parts  of  the  small  intestine,  at  other  times  almost 
wanting;  usually  they  are  few  in  the  upper  and  numerous  in  the  middle  and 
lower  parts  of  the  tube.  They  appear  as  small  whitish  elevations,  spherical 
or  pyriform  in  shape  and  from  .2-2.5  "''"i-  i"  diameter.  Villi  and  crypts  of 
Lieberkiihn  are  wanting  over  the  prominence  of  the  nodules  (Fig.  210). 


THE   SMALL    INTESTINE. 


169 


Solitary 
nodule 


Villus 


^i 


In  structure  the  solitary  nodules  correspond  to  lymph-nodules  in  other 
localities,  consisting  of  a  capsule  of  fibrous  tissue  enclosing  the  delicate  reticu- 
lum which  supports  the  lymphocytes  within  its  meshes.  Within  the  larger 
nodules,  spherical  or  ellip- 
soidal germ-centres  are 
present;  they  are,  how- 
ever, not  constant,  being 
present,  as  a  rule,  in  young 
subjects,  but  often  absent 
in  older  ones.  Each  nod- 
ule is  surrounded  by  a  rich 
network  of  small  blood- 
vessels, from  which  fine 
capillaries  penetrate  the 
lymphoid  mass  (  Fig.  151). 
Definite  lymph-paths  are 
absent  within  the  nodules, 
although  a  plexus  of  lym- 
phatics surrounds  their 
exterior. 

The  aggregated  nod- 
ules or  Peyer's  patches 
are  collections  of  simple 
nodules,  the  individual  nodules  being  blended  by  intervening  lymphoid  tissue 
(Fig.  211).  They  are  present  in  the  lower  half  of  the  small  intestine,  especially 
in  the  ileum,  but  exceptionally  are  found  as  high  as  the  beginning  of  the 
jejunum.     The  patches  appear  as  slightly  raised  elongated  oval  areas.     They 

Submucosa  supporting  mucosa  ;  ■ 


Orifice  of 

Lieberkijhn's 

gland 


-^i-: 


Fig.  210. — Surface  view  of  mucous  membrane  of  small  intes- 
tine (ileum),  showing  villi,  glands  and  solitary  lymph-nodules. 
X30. 


^atg' 


\\\S 


oi  pa' 


XcVv 


Solitary  nodules 


Fig.  211. — Transverse  section  of  ileum,  showing  a  Peyer's  patch  cut  across.      ■   8. 

usually  number  about  thirty,  although  as  few  as  eighteen  or  as  many  as  eighty 
have  been  counted.  Their  length  is  ordinarily  from  1-4  cm.  and  their  breadth 
from  6—16  mm.  Each  patch  contains  from  20-30  ovoid  individual  lymph- 
nodules  which,  when  well  developed,  occupy  both  the  mucous  and  submucous 
coats,  their  smaller  ends  almost  reaching  the  epithelium  and  their  bases  the 
muscular  tunic.  The  villi  and  crypts  of  Lieberkiihn  are  present  over  the 
areas  between  the  nodules,  although  less  developed  than  beyond  the  patch. 


NORMAL    HISTOLOGY. 


Mucous 
coat 


Submucous 
coat 


In  Structure,  the  component  lymph-nodules  correspond  to  the  solitary  nodules, 
the  aggregated  nodules  being  blended  into  a  continuous  mass  by  the  less  dense 
lymphoid  tissue  filling  the  spaces  between  the  individual  nodules.  The  entire 
patch  is  defined  from  the  surrounding  tissues  by  an  imperfect  fibrous  capsule. 
The  submucous  coat  of  the  small  intestine,  although  lax,  does  not 
allow  displacement  of  the  plicse  circulares,  except  in  the  lower  part  of  the 
tube.      In  addition  to  most  of  Brunner's  glands  and  the  lymph-nodules,  the 

submucosa  contains  blood- 
vessels and  lymphatics  of 
considerable  size  and  the 
nerve-plexus  of  Meissner. 

The  muscular  coat, 
about  .4  mm.  thick,  consists 
of  an  inner  circular  and  an 
outer  longitudinal  layer  of 
involuntary  muscle.  The 
circular  stratum  is  some  two 
or  three  times  as  thick  as 
the  longitudinal  one  and  is 
the  more  regular  in  arrange- 
ment. The  thin  longitudi- 
nal layer  is  often  imperfect, 
especially  along  the  attach- 
ment of  the  mesentery.  The 
entire  muscular  coat  dimin- 
ishes in  thickness  towards 
the  lower  end  of  the  small 
intestine. 

The  serous  coat,  with 
the  exception  of  that  of 
the  duodenum,  completely 
invests  the  gut  except  along 
the  line  of  attachment  of  the 
mesentery,  where  the  two 
layers  of  peritoneum  diverge, 
leaving  a  non-serous  area 
between  them  for  the  passage  of  blood-vessels,  lymphatics  and  nerves. 
In  structure  the  serous  coat  of  the  intestine  corresponds  to  that  of  the 
stomach,  and  includes  essentially  the  fibro-elastic  connective  tissue  stroma, 
covered  on  the  free  surface  with  mesothelium. 

The  blood-vessels  supplying  the  small  intestine  reach  the  walls  of  the 
tube  between  the  peritoneal  folds  constituting  the  mesentery.  After  sending 
branches  to  the  serous  coat,  the  arteries  penetrate  the  muscular  tunic  (to  the 
outer  part  of  which  twigs  are  given  in  passing)  to  gain  the  submucosa. 
Within  the  latter,  additional  twigs  are  given  to  the  muscular  coat,  while 
others  supply  the  glands  and  lymph-nodules  lying  in  this  tunic.  Larger 
branches  pass  from  the  vessels  of  the  submucosa  into  the  mucous  membrane, 
some  to  break  up  into  capillaries  forming  networks  around  the  gland-tubules 
and  others  to  supply  the  villi.  Each  villus  receives  from  one  to  three  arteri- 
oles, which  resolve  into  capillaries  occupying  the  peripheral  part  of  the 
stroma.  The  blood  is  returned  by  a  single  axial  vein  which  traverses  the 
projection  and  becomes  tributary  to  the  larger  venous  stems  within  the  sub- 
mucous coat,  as  do  the  other  veins  of  the  tunica  propria  that  commence  near 


-Transverse  section  of  injected  small  intestine,  show- 
ing general  distribution  of  vessels.     X  45. 


THE   LARGE    INTESTINE. 


171 


the  epithelium.  The  veins  within  the  subnaucosa  accompany  the  arteries 
throug-h  the  muscular  coat  and  unite  into  the  emergent  venous  channels  that 
course  with  the  arteries  between  the  peritoneal  folds. 

The  lymphatics  of  the  small  intestine,  long  known  as  the  hiciea/s  on 
account  of  their  milky  appearance  when  containing  finely  divided  particles 
of  fat,  begin  as  the  absorbent  vessels  of  the  villi.  In  addition,  lymph-chan- 
nels form  a  plexus  within  the  tunica  propria  in  the  neighborhood  of  the 
muscularis  mucosae,  from  which  tributaries  pass  to  the  larger  submucous 
plexus.  The  latter  is  characterized  bv  irregular  contours,  due  to  the  dila- 
tations associated  with  the  numerous  vah-es.  The  emergent  lymphatics 
penetrate  the  muscular  coat  and,  within  the  serous  tunic,  unite  into  larger 
trunks  that  pass  to  the  lymph-nodes  between  the  peritoneal  folds;  from  these 
smaller  nodes  lymphatics  converge  to  the  larger  mesenteric  lymph-nodes. 

The  nerves  supplying  the  small  intestine  are  derived  from  the  solar 
plexus,  and  include  both  medullated  and  nonmedullated  fibres,  the  last  being 
chiefly  from  the  svmpathetic.  In  their  distribution,  they  closely  follow  the 
arrangement  observed  in  the  stomach  (page  163),  including  the  plexus  of  Auer- 
bach  and  of  Meissner  and,  additionallv,  a  plexus  of  nonmedullated  fibres 
within  the  \\\\\. 

THE   LARGE    INTESTINE. 

The  large  intestine,  subdivided  into  the  caecum,  the  colon  and  the  rectum, 
measures  about  i .  5  meters  or  nearly  5  ft.  in  length.    As  other  parts  of  the  intesti- 


Lieberkiihn's  glands" 


Solitary  lymph-nodule: — 


Mucous  coat 


^^. 

■ 

1 

m 

^Ki; 

■^■^ 

Submucous" 
coat 


Circular- 
muscle 


J.ong-itndinal' 

muscle 
Serous  coat- 


FiG.  213. 


-Longitudinal  section  of  large  intestine  (ascending  colon),  showing  the  general  arrangement  of 
the  coats  and  a  solitary  lymph-nodule.     X  30. 


naltube,  it  consists  of  four  coats — the  mucous,  submucous,  muscular  and  serous. 

The  mucous  coat  of  the  large  intestine  agrees  in  its  essential  structure 

with  that  of  the  small  gut,  consisting  of  a  tunica  propria,  resembling  lymphoid 

tissue,  co\'ered  by  a  single  layer  of  columnar  epithelium  exhibiting  a  cuticular 


172  NORMAL   HISTOLOGY. 

border.      It  differs,  however,  in  having  neither  villi  nor  plicae  circulares,  and, 

in  consequence,  lacks  the  velvety  appearance  of  the  small  intestine,  the  mucous 

surface  being  smooth,  although  thrown  into  folds  and  pouches  by  modifications 

in  other  coats.      The  muscularis  mucosae  is  less  regular  in  its  development, 

being  feebly  represented  in  the  colon  and  exceptionally  thick  in  the  rectum. 

The  crypts  of  Lieberkuhn  resemble  those  of  the  small  intestine,  but  are 

larger  (.4-5  mm.  long)  and  less  interrupted.      Within  the  rectum  they  may 

attain  a  length  of  .  7  mm.     The  Hning  of  the  crypts  is  conspicuous  on  account 

of  the  abundance  of  goblet-cells,  which  in  the  middle  and  upper  parts  of  the 

tubules  almost  replace  the  ordinary  epithelial  elements.      As  in  the  small 

intestine  so  here,  mitotic  figures  are  often  seen  in  the  cells  lining  the  crypts, 

A  the  new  elements    so  arising  being  eventually 

,_,^     pushed  to  the  surface  to  replace  the  old  ones 

that  are  disappearing. 

The   lymphoid    tissue    occurs    as    solitary 

nodules  only,  Peyer's  patches  being  absent  in 

the    large   intestine.     The   lymph-nodules    are 

largest  and  most  plentiful  in  the  caecum  and  in 

the  vermiform  appendix ;  in  the  latter  situation 

they  are  so  numerous 

/.v.„  '    -  '^  L"  that  in  places  they  form 

''^''~*'^>f.r'/t>j'' '^       '#*"       an    almost    continuous 


mass  of  lymphoid  tis- 
sue. In  the  colon  the 
nodules  are  less  abun- 
dant, but  in  the  rectum 
;''/.^^'^^"%H}'',z.%^^S-'^1^C''  they  are  again  numer- 
'iX  ^^i* v^yf'.'J- '"  '  ous.      They  are  gener- 

*•  ^>"j^i^'t/<<''-- '"  ^lly  of  larger  size  (1.5- 

'■'•'*'.•„-;•■"  3    mm.)    than    in    the 

Fig.  214. — Portion  of  mucosa  of  large  intestine,  showing  crypts  of     Small  intestine. 
Lieberkuhn  cut  lengthwise  {A)  and  crosswise  (B);  epithelial  elements  The    SubmUCOUS 

contain  mucus  and  are  "goblet-cells."     X  i6o.  ^11  11 

coat  closely  resembles 
the  similar  fibro-elastic  connective  tissue  tunic  of  the  small  intestine,  and 
allows  fairly  free  play  of  the  mucous  membrane.  In  addition  to  the  blood- 
\essels,  lymphatics  and  nerve-plexus  of  Meissner,  it  contains  the  deeper  and 
more  expanded  portions  of  the  solitary  lymph-nodules. 

The  muscular  coat  includes  a  thicker  internal  layer  of  circular  fibres 
and  an  external  one  of  longitudinal  fibres;  the  latter,  however,  are  not 
uniformly  distributed,  but,  in  most  places,  are  collected  into  three  bands, 
the  tcenice,  between  which  the  longitudinal  muscular  coat  is  extremely  thin 
or  imperfect.  These  bands  are  shorter  than  the  layers  of  the  intestinal  wall 
internal  to  them,  and  are  responsible  for  the  characteristic  sacculation  of  the 
large  intestine.  The  circular  muscle  increases  markedly  towards  the  lower 
end  of  the  rectum,  and  in  the  anal  canal  becomes  augmented  into  a  sheet  of 
in\oluntary  muscle,  some  4  mm.  thick,  known  as  the  internal  sphincter. 

The  serous  coat  is  incomplete  in  certain  parts  of  the  large  intestine 
owing  to  secondary  changes  during  development  and  growth.  In  structure 
it  corresponds  to  the  peritoneal  investment  of  other  parts  of  the  alimentary 
tract  (page  175).  The  characteristic  little  fringes  or  bags,  the  appc7idiccs 
epiploicce,  that  are  attached,  particularly  along  the  median  aspects  of  the 
ascending  and  descending  colon  and  on  the  lower  side  of  the  transverse 
colon,  consist  of  pouches  of  peritoneum  filled  with  adij^ose  tissue  TFig.  21S). 


THE   LARGE   INTESTINE. 


173 


Mucosa 


Submucosa 


The  blood-vessels,  lymphatics  and  nerves  of  the  lari^e  intestine  follow, 
in  the  details  of  their  distribution,  the  general  plan  described  in  connection 
with  the  small  intestine  (page  170). 

The  ileo-colic  valve,  guarding  the  entrance  of  the  ileum  into  the 
large  intestine,  results  from  the  thrusting  of  the  small  gut  into  the  large, 
during  foetal  life,  in  such  a  way  that  originally  all  layers  of  the  intestinal  wall 
are  involved.  Where  the  two  serous  coats  come  into  contact,  the  meso- 
thelium  disappears  and  the  permanent  union  is  effected  by  fibro-elastic  tissue 
and  secondarily  developed  longitudinal  muscle.  Although  both  layers  of 
the  original  muscular  coat  are  carried  into  the  folds  of  the  valve,  it  is  the  cir- 
cular muscle  that  undergoes  marked 
thickening  and  forms  the  efficient 
sphincter  guarding  the  opening.  The 
mucosae  covering  the  two  sides  of  the 
crescentic  valve-folds  differ,  that  con- 
tinued from  the  ileum  possessing  villi 
which,  as  rudimentary  elevations,  con- 
tinue almost  to  the  margin  of  the  folds. 

The  vermiform  appendix, 
the  slender  worm-like  appendage  at- 
tached to  the  caecum,  about  8.4  cm. 
(3^  in. )  long  and  6  mm.  in  diameter, 
contains  all  the  coats  of  the  large  in- 
testine. The  mucous  coat  is  thrown 
into  longitudinal  folds  and  encloses 
a  narrow  irregular  lumen.  In  its  gen- 
eral structure  it  corresponds  to  the 
mucosa  of  the  large  intestine,  but  is 
infiltrated  to  an  unusual  degree  with 
lymphocytes.  These  are  collected 
into  many  lymph-nodules,  with  germ- 
centres,  the  lymphoid  tissue  being  so 
abundant  that  it  often  almost  encircles 
the  appendix  as  a  continuous  mass. 
The  crypts  of  Lieberkiihn  contain  an 
unusually  large  number  of  goblet- 
cells;  these  are,  however,  few  on  the 
free  surface.  The  inner  circular  mus- 
cle is  about  twice  as  thick  as  the  ex- 
ternal longitudinal  layer.  The  lym- 
phoid tissue  of  the  vermiform  appendix  is,  as  elsewhere,  most  developed  in 
childhood  and  tends  to  atrophy  in  middle  life.  Along  with  such  atrophy,  the 
walls  of  the  appendix  manifest  a  disposition  to  adhere,  more  or  less  obliterating 
the  lumen  of  the  tube.  In  consequence  of  these  changes,  after  the  thirty- 
fifth  year  the  appendix  often  exhibits  variations  from  the  normal  condition. 

The  rectum,  including  the  anal  canal,  presents  modifications  calling 
for  passing  notice.  The  crypts  of  Lieberkiihn  are  especially  large  (.7  mm. 
in  length),  although  less  numerous,  and  do  not  entirely  disappear  until  the 
transition  of  the  columnar  to  the  stratified  squamous  epithelium  has  been 
reached  or,  sometimes,  even  slightly  passed.  This  transformation  begins  at 
the  upper  ends  of  the  vertical  mucosa-folds,  the  rectal  cobanns  ox  columns  of 
Morgagni,  that  surround  the  anal  canal  and  contain  strands  of  muscle;  at 
the  level  of  the  crescentic  folds,  the  anal  valves,  connecting  the  bases  of  the 


Longitudinal 
muscle 


Serosa 


Fig.  215. — Transverse  section  of  injected  large 
intestine,  showing  distribution  of  arteries  to  the 
coats.    X  20. 


174 


NORMAL    HISTOLOGY. 


columns,  the  mucous  membrane  is  replaced  by  the  skin  lining  the  lower 
segment  of  the  anal  canal.  The  surface  of  the  rectal  mucosa  is  punctated 
with   minute  tubular  depressions,    the  rectal  pits  of    Cunningham,    at  the 


■^i'v 


Lumen  of 

caecum 

Orifice  of 

appendix 


"■1 


■^ 


Circular  muscle 


Lymph-nodules 


:^. 


Lieberkijhn's 
■Trlands 


Mucous  coat 


Croups  of 
~rat-cells 


:f'' 


Submucous  tissue  — "      — «:2>'-'     ^*=.;i5  ^  j 

Fig.  2i6. — Longitudinal  section  through  junction  of  vermiform  appendix  and  caecum.     X  lo. 

bottom  of  each  of  which  is  an  accumulation  of  lymphoid  tissue  resembling  a 
lymph-nodule.     The  submucous  coat  is  lax  and  contains  the  extensive  hemor- 


Longitudinal 
"muscle 


Circular  muscle 


Glands 
Lumen  ■ 

Lymph-nodule ' 


4m^ '"' 


Lymphoid 


Obliquely  cut 
glands 


IS 


Submucous 
^'oat 


'vV 


^f 


-fV' 


Fig.  217.— Transverse  section  of  vermiform  appendix.     X  10. 

rhoidal  plexus  of  veins.  In  addition  to  the  temporary  folds,  the  rectal 
mucous  membrane  presents  usually  three  crescentic  shelf-like  projections, 
the   rectal  valves,   that  are  ineffaceable.      These   plicae  are  formed  by  the 


THE    PERITONEUM. 


175 


infolding-  of  the  mucosa,  submucosa  and  greater  part  of  the  muscuhir  tunic, 
a  portion  of  the  longitudinal  muscle  passing  over  the  creases  externally. 
The  shortness  of  the  muscular  bands,  into  which  the  longitudinal  muscle  is 
condensed  in  front  and  behind,  serves  to  maintain  these  trans\erse  folds. 
Where  the  peritoneum  is  wanting,  as  it  is  except  over  part  of  the  anterior 
aspect  of  the  rectum,  the  serous  coat  is  replaced  by  a  fibrous  one  composed 
of  fibro-elastic  tissue. 

THE    PERITONEUxM. 

The  peritoneum,  the  serous  membrane  lining  the  abdominal  cavity  and 
covering  more  or  less  completely  the  therein  contained  organs,  consists  of  a 
connecti\'c  tissue  stroma  and  the  surface  layer  of  mesothelhim.  The  latter 
is  a  single  layer  of  plate-like  cells  (Fig.  25)  irregularly  polygonal  in  form 
and  of  varying  size,   whose  contours  are  mapped  __^ 

out,  after  staining  with  silver  nitrate,  by  delicate 
sinuous  dark  lines  that  correspond  with  the  particles 
of  reduced  silver  in  the  intercellular  cement-sub- 
stance. Each  cell  encloses  an  oval  flattened  nucleus, 
usuallv  somewhat  eccentrically  placed,  that  is  almost 
in\isible  until  tinged  with  some  appropriate  dye. 
The  size  and  form  of  the  mesothelial  plates  varv 
much  with  the  tension  to  which  they  are  subjected; 
when  unduly  stretched,  they  are  often  imperfect  or, 
indeed,  displaced. 

The  stroma  consists  of  a  feltwork  of  con- 
nective tissue  bundles  of  variable  hneness,  those 
of  the  parietal  being  commonly  more  robust  than 
those  of  the  visceral  peritoneum.  The  deeper  part 
of  the  stroma  contains  numerous  elastic  fibres, 
which  are  most  abundant  and  de\eloped  in  the 
parietal  sheet,  where  they  form  a  distinct  network. 
Seldom  in  the  mesenteries  but  constantly  in  the 
omenta,  the  stroma  undergoes  partial  absorption, 
whereby  larger  or  smaller  o-\^&mw^%,  fenestra,  result 
(Fig.  38).  In  this  manner  what  originally  was  a 
continuous  sheet  becomes  a  fenestrated  membrane, 
over  which  the  mesothelium  stretches  as  an  un- 
broken covering,  investing  the  trabeculae  as  well 
as  the  parts  still  retaining  the  character  of  membranes.  The  nuclei  of  the 
connective  tissue  cells  and  those  of  the  mesothelial  plates  are  seen  inter- 
mingled. Although  all  the  important  peritoneal  folds,  as  the  mesenteries, 
omenta  and  many  of  the  so-called  ligaments  of  the  viscera,  theoretically 
include  two  layers  of  serous  membrane  and  an  intervening  layer  of  connective 
tissue  prolonged  from  the  body-wall,  in  which  course  the  \essels  and  nerv'es 
supplying  the  organs,  such  duplicatures  consist  essentially  of  a  general 
connective  tissue  stroma-layer  covered  on  each  side  by  a  stratum  of  meso- 
thelium. Wherever  two  peritoneal  surfaces  are  brought  into  permanent 
contact,  the  mesothelium  disappears  and  the  serous  character  of  the  attach- 
ment is  lost,  the  union  henceforth  being  one  of  fibrous  tissue.  Where  readily 
mo\'able,  as  o\'er  most  parts  of  the  abdominal  and  pehic  walls  and  many  folds, 
the  attachment  of  the  peritoneum  to  the  subjacent  parts  is  effected  by  a  layer 
of  fat-laden  subserous  tissue.  This  fibro-elastic  layer  varies  in  thickness, 
but  in  many  places,  as  over  the  liver,  stomach  or  intestine,  where  the  perito- 


Artery 


Fig.  218. — Lon^^itudinal  sec- 
tion of  an  epiploic  appendage. 
X  22. 


176 


NORMAL    HISTOLOGY. 


neum  is  intimately  attached,  the  subserous  tissue  is  so  reduced  as  to  be  practi- 
cally wanting;.  In  certain  localities,  conspicuously  in  the  broad  ligament  of 
the  pelvis,  the  subserous  tissue  contains  strands  of  unstriped  muscle. 

The  blood-vessels  supplying  the  peritoneum  itself  are  meagre  in  size 
and  number.  The  lymphatics  include  a  superficial  network  beneath  the 
mesothelium  and  a  deeper  plexus  of  lymph-channels  within  the  stroma.  The 
nerves  include  both  meduUated  and  nonmedullated  fibres,  the  latter  being 
destined  for  the  walls  of  the  blood-vessels.  The  sensory  fibres  supplying  the 
parietal  peritoneum  in  many  cases  are  connected  with  lamellated  corpuscles 
and  end-bulbs  (Fig.   ii6). 

THE   LIVER. 

The  liver,  the  largest  gland  in  the  body,  consists  of  very  delicate  gland- 
ular tissue  disposed  around  the  ramifications  of  the  portal  vein.  Developed 
in  the  primitive  anterior  mesentery,  its  connective  tissue  (mesodermic)  ele- 


Central  vein 


Blood-capillaries 

Portal  vein 

Hepatic  artery. 

Bile-vessel. 


Portal  vein 


Bile-capillaries 


Blood-capillaries 


ortal  vein 


Sublobular  branch  of  hepatic  vmp  \ 

U-  -^h  '-'^ 

Fig.  2T9.— Diagram  of  hepatic  lobule;  portions  of  surface  and  of  transverse  and  longitudinal  sections 
of  the  lobule  are  represented.  The  branches  of  the  portal  vein  are  purple ;  of  the  hepatic  artery,  red  ;  of 
the  hepatic  veins,  blue;  of  bile-ducts,  yellow;  the  intralobular  bile-canaliculi are  black. 


ments  have  a  common  origin  with  the  diaphragm,  while  its  duct  and  glandular 
elements  are  derived  from  a  sprout  from  the  duodenum.  Hence  the  liver  is  an 
outgrowth  and  appendage  of  the  alimentary  tube.  Its  peculiar  shape  is  due 
chiefly  to  the  pressure  of  surrounding  organs,  since  its  tissue  is  so  plastic  as  to  be 
moulded  by  them.  The  liver  weighs  from  I450-I750gm.,  approximately  3-3% 
lbs. ,  and  in  the  adult  contributes  about  one  fortieth  of  the  entire  body  weight. 
In  its  fundamental  arrangement,  the  liver  corresponds  to  a  highly 
modified  compound  tubular  gland.  Early  in  foetal  life,  however,  the  termi- 
nal divisions  of  the  tubules  unite  to  form  networks,  after  which  the  tubular 


THE    LIVER. 


177 


character  of  the  Hver  becomes  progressively  more  masked  by  the  inter- 
growth  of  the  cell-cords  and  the  large  veins.  The  glandular  tissue  is  subdi- 
vided by  connective  tissue  into  small  cylindrical  masses,  the  lobules,  which 
on  the  surface  of  the  organ  are  seen  as  little  polygons,  1-2  mm.  in  diameter. 
This  interlobular  tissue  is  continuous  with  the  fibrous  envelope,  or  capsule, 
that  invests  the  exterior  of  the  liver,  at  the  transverse  fissure  being  prolonged 
as  the  capside  of  Glisson  into  the  organ  in  company  with  the  interlobular 
vessels.  The  distinctness  with  which  the  lobules  are  defined  depends  upon 
the  amount  of  the  interlobular  tissue.  This  is  notably  abundant  in  the  hog's 
liver,  in  which  the  lobules  appear  as  sharply  marked  polygonal  areas.  In 
the  human  liver,  on  the  contrary,  the  interlobular  tissue  is  very  meagre, 
the  lobules,  in  consequence,  being  poorly  defined  and  uncertain  in  outline 
(Fig.  221). 

The  Blood-Vessels  of  the  Lobule. — The  arrangement  of  the  blood- 
vessels is  the  salient  feature  in  the  architecture  of  the  fully  formed  hepatic 


Portal  vein 


Fig.  220. — Section  of  liver  injected  from  hepatic  vein,  showing  intralobular  capillary  network.     X  loo. 

lobule.  The  divisions  of  the  portal  vein — the  vessel  bringing  blood  to  the  liver 
for  its  function — enter  at  the  transverse  fissure  and  break  up  into  branches 
which  ramify  within  the  interlobular  tissue  (capsule  of  Glisson)  and  encircle 
the  lobules.  These  interlobular  veins  give  off  numerous  small  branches  that 
enter  the  periphery  of  the  lobules  to  resolve  at  once  into  the  intralobular 
capillary  network.  The  general  disposition  of  this  network  is  radial,  the 
capillaries  converging  towards  the  middle  of  the  lobule  where  they  join  to 
form  and  empty  into  the  central  or  iritralobular  vein.  The  course  of  the 
latter  corresponds  with  the  long  axis  of  the  lobule,  hence,  in  cross-sections  . 
of  the  lobule,  the  central  vein  appears  as  a  transversely  cut  channel  towards"*" 
which  the  capillaries  converge  (Fig.  220^.  The  capillary  netiuork  within 
the  lobule  is  made  up  of  vessels  usually  about  10  //  in  diameter,  the  widest 
capillaries  (20  //)  being  in  the  immediate  vicinity  of  the  afferent  and  efferent 
veins.  The  meshes  of  the  capillary  network  vary  from  15-45  /-'-  i^^  their 
greatest  dimension,  those  at  the  periphery  being  broader  and  more  rounded, 
while  those  near  the  centre  of  the  lobule  are  narrower  and  more  elongated. 


lyS  NORMAL   HISTOLOGY. 

The  central  vein  traverses  the  axis  of  the  lobule,  enlarging  as  it  proceeds, 
to  the  base,  as  the  side  of  the  lobule  through  M^hich  the  vein  escapes  is  termed. 
The  central  vein  begins  usually  about  midway  between  the  base  and  the 
opposite  border  of  the  lobule  and  is  formed  by  the  confluence  of  the  capillary 
tributaries.  Immediately  on  emerging  from  the  lobule,  the  central  vessel 
opens  into  a  sublobular  vein,  which  runs,  in  a  general  way,  at  right  angles 
to  its  intralobular  tributaries  and  along  and  beneath  the  bases  of  the  lobules. 


Central  (intra- 
lobular) vein 


V 


^<         '*:,'.  Interlobular 

^\V,\  He  —connective 

J%^",  Sgia-  tissue 


Fig.  221.— Section  of  uninjected  liver,  showing  the  general  arrangement  of  the  lobules,  interlobular  and 

intralobular  vessels.    X  120. 

The  sublobular  veins  are  thus  surrounded  by  the  bases  of  the  lobules,  a 
single  central  vein  returning  the  blood  from  each.  The  sublobular  veins 
join  to  form  larger  trunks,  which  in  turn  unite  and  constitute  the  branches  of 
the  hepatic  veins,  the  large  venous  channels,  commonly  several,  that  carry 
the  blood  from  the  liver  into  the  inferior  vena  cava. 

The  Liver-Cells. — The  meshes  of  the  interlobular  capillary  network 
are  occupied  by  the  hepatic  cells,  the  bile-capillaries  and  a  meagre  amount 
of  delicate  connective  tissue.     The  liver-cells  are  arranged  as  cords  or  trabec- 


THE    LIVER.  179 

ulae  which  conform  in  their  general  disposition  and  shape  to  the  intercapillary 
spaces  which  they  fill.  When  isolated,  the  liver-cells  present  a  polygonal 
outline  and  measure  from  15-25  11.  in  their  longest  dimension.  Each  cell 
comes  into  contact  with  from  six  to  nine  other  ones,  the  surfaces  of  contact 
being  plane  from  mutual  pressure.  Always  one  side,  often  more  than  one, 
exhibits  a  shallow  concavity  that  indicates  the  surface  of  former  contact  with 
a  capillary.  The  cells  lie  against  at  least  one  capillary  and  sometimes  several, 
this  relation  depending  upon  the  size  of  the  blood-channel.  The  liver-cell 
consists  of  finely  granular  protoplasm,  which  at  times  exhibits  a  differentiation 
into  an  outer  and  inner  zone.  It  is  without  a  cell-membrane,  although  the 
cytoplasm  is  condensed  at  the  periphery.  The  spherical  nucleus  contains 
relatively  little  chromatin  and  usually  a  nucleolus.  Occasional  cells  are 
conspicuous  on  account  of  their  large  size  and  unusually  large  nucleus; 
such  elements  probably 

vmdergo  direct  division  ^^^ 

and  produce  the  double  /_. '^^'^V'"'     ^^  "    '*'" 

nucleated  cells  constant-  -^-V  ' 

ly  encountered    in    sec-  L-  T,^^ 

tions    of    normal    liver.  ^  k^  -   -^^ 

Centrosomes    have    also  -  *^  i®> 

been  observed.    Particles 

of  glycogen,  minute  oil      /    '••  .  , 

drops,   and  granules   of       ^  -^^  '  . 

bile-pigment    are    fairly  ^  C 

constant  inclusions.  Fat-  ^     -3 

containing  cells  are  most 

abundant  at  the  periph-  .  J  i 

ery  of  the  lobule,  those 
containing  pigment  par-        '     ^ 

tides  near  the  centre.  ^'^*-     ^  „'    __  '-^.''^ 

The    Bile-Capil-  "         ^^  '     ^  ^ 

lanes       T^ho^iO    minute 

,     '  .  ,  Fig.  222. — Section  of  uninjected  liver,  showing  cords  of  hepatic  cells 

canals,    representmg    the  between  the  capillary  blood-vessels.    X  400. 

lumina  of  ordinary  tubu- 
lar glands,  form  a  network  of  intercommunicating  channels  throughout 
the  lobule,  closely  related  to  the  liver-cells.  Instead  of  the  arrangement 
usual  in  glands,  where  several  secreting  cells  border  the  gland-lumen  each 
with  a  single  surface,  in  the  exceptional  case  of  the  liver  the  excretory 
channels  are  bounded  by  the  opposed  surfaces  of  only  two  cells,  the 
bile-capillary  occupying  but  a  small  part  of  these  surfaces,  which  it 
models  with  a  narrow  groove.  Moreover,  the  canaliculi  are  not  limited  to  a 
single  surface  on  each  cell,  for  they  are  found  between  all  surfaces  where 
two  liver-cells  are  directly  in  contact.  Hence,  each  hepatic  cell  is  in  imme- 
diate relation  with  a  number  of  bile-capillaries.  The  latter,  however,  never 
lie  on  the  surfaces  of  the  liver-cells  directed  towards  the  blood-channel,  the 
bile-canaliculus  never  separating  the  blood-capillary  and  the  liver-cell. 
Whilst  the  dominating  direction  of  the  bile-capillaries  is  radial  and  corre- 
sponds to  the  general  disposition  of  the  trabeculae  of  hepatic  cells,  this  radial 
arrangement  is  converted  into  a  network  by  numerous  cross-branches  (Fig. 
224).  The  resulting  meshes  agree  in  size  and  form  with  the  individual  liv^er- 
cells,  which  often  appear  surrounded  by  the  bile-capillaries.  The  latter 
possess  no  walls  other  than  the  substance  of  the  liver-cells  between  which 
they  lie.     The  diameter  of  the  minute  biliary  canals  (1-2  [j.  )  remains  con- 


i8o 


NORMAL    HISTOLOGY. 


stant  throughout  the  lobule  until  the  canaliculi  reach  the  margin.      Here  the 
liver-cells  abruptly  diminish  in   size  and  become  continuous  with  the  low 


-'\Jii. 


m 


hJ' 


^S:#c^^% 


-  Blood-capillary 


-Bile-capillary 


Liver-cell 


(>;ii|f^;v  ^^.uv^ 


Fig.  223— Section  of  liver  in  which  both  the  blood- and  bile- capillaries  have  been  injected;  the  biliary 
channels  surround  the  individual  liver-cells.     X  300. 

cuboidal  cells  that  line  the  excretory  tubes  passing  from  the  lobule  into  the 
surrounding  connective  tissue  to  become  tributary  to  the  larger  interlobular 


A. 


-3r 


J     .     ^ 


•sfc'^ 


Fig.  224. — Section  of  liver  treated  vkfith  Golg-i 
method,  showing  part  of  the  intralobular  network  of 
bile-capillaries.     X  200. 


Fig.  225. — Artificially  digested  section 
of  liver,  showing  supporting  interlobular 
fibrous  tissue  (fjelow)  and  intralobular 
reticulum  (above).     X  200. 


bile-ducts.      The  ultimate  relations  between  the  bile-capillaries  and  the  liver- 
cells  is  still  a  subject  of  discussion.      According  to  some,  extensions  of  the 


thp:  biliary  passages. 


l8l 


capillaries  norniallv  exist  within  the  substance  of  the  cells,  thus  forming 
intracellular  sccrction-canalicidi.  The  latter  are  sometimes  pictured  as 
ending  in  minute  dilatations,  the  secretion-vacuoles.  It  is  highly  probable  that 
such  appearances,  while  not  artefacts,  at  least  depend  upon  particular  condi- 
tions of  secretory  activity  and  are,  therefore,  not  constant  details  of  the 
hepatic  cells. 

The  intralobular  connective  tissue  is  very  meagre  in  amount  and 
consists  of  delicate  prolongations  of  the  interlobular  fibrous  tissue  along  the 
blood-capillaries.  The  tissue  occurs  only  between  the  blood-channels  and 
the  cells  and  never  between  the  latter.  Although  present  in  some  quantity 
immediately  around  the  central  vein,  in  other  parts  of  the  lobule  it  is  repre- 
sented by  lattice-works  of  fibres  which  surround  the  capillaries.  The  fibres 
are  not  elastic  in  nature,  but  correspond  most  closely  to  modified  white  fibres. 


«^' 


Portal  vein 


/ 


Bile-duct    ' 
Hepatic  in(.r>- 


Interlobular  Lonnective- 
tissue 


'i' 


Hepatic  cells 


Fig.  226. — Section  of  liver,  showing  interlobular  tissue  and  vessels.     X  160. 

the  entire  intralobular  connective  tissue  belonging  to  the  variety  known  as 
reticulum.  The  small  spindle  or  stellate  elements  seen  in  gold  preparations, 
known  as  the  ce//s  of  Kupjfer,  have  been  shown  to  belong  to  the  capillary  wall 
(perhaps  distorted  endothelial  cells)  and  not  to  the  perivascular  fibrous  tissue. 

The  Biliary  Passages. — The  interlobular  bile-ducts,  which  re- 
ceive the  canals  that  pierce  the  periphery  of  the  lobule  as  outlets  of  the 
intralobular  network,  accompany  the  branches  of  the  portal  vein  and  of  the 
hepatic  artery  within  the  connective  tissue  between  the  lobules.  The  ducts, 
from  30-50  ;/  in  diameter,  form  a  network  over  the  exterior  of  the  lobule 
and  possess  walls  consisting  of  a  delicate  fibro-elastic  coat,  in  the  smallest 
tubes  little  more  than  a  basement  membrane,  lined  with  low  columnar  epi- 
thelium continuous  with  the  cuboidal  cells  clothing  the  emergent  canals. 
The  perilobular  ducts  are  tributaries  of  larger  bile-\essels  which  increase  in 
diameter  as  they  pass  towards  the  transverse  fissure. 

The  large  ducts  join  into  two  main  lobar  trunks,  by  whose  union,  within 
or  just  beyond  the  transverse  fissure,  the  hepatic  duct  is  formed,  a  tube 
from  4-6  mm.  in  diameter  and  about  2.5  cm.  long.  Its  walls  include  a  dense 
fibro-elastic  tunica  propria,  covered  with  a  single  laver  of  columnar  epithelium 


I82 


NORMAL   HISTOLOGY. 


and  beset  with  scattered  small  tubular  glands.  These,  as  well  as  the 
surface  epithelium,  contain  many  goblet-cells  producing  a  mucous  secretion. 
Bundles  of  unstriped  muscle  occur  within  the  deeper  parts  of  the  tunica 
propria;  they  are  neither  numerous  nor  regularly  disposed  in  definite  layers, 
the  chief  longitudinal  ones  being  supplemented  by  circular  and  oblique  bun- 
dles. As  the  duct-system  is  followed  into  the  capsule  of  Glisson,  the  muscle 
disappears  from  the  walls  of  all  but  the  larger  interlobular  bile-vessels,  while 
the  fibro-elastic  coat  also  gradually  diminishes.  Apart  from  the  reduction 
in  height  of  the  cells,  the  lining  of  the  duct's  retains  throughout  its  character 
of  simple  columnar  epithelium,  thus  affording  a  ready  means  of  distinguishing 
the  bile-ducts  from  the  blood-vessels  as  they  course  together  between  the 
lobules.  The  existence  of  former  masses  of  hepatic  tissue,  which  have  dis- 
appeared during  development  and  growth,  is  indicated  by  the  blind  ducts, 
known  as  the  vasa  abcT'rantia,  found  outside  the  liver-substance  along  the 
left  border  of  the  liver,  around  the  inferior  vena  cava  and  in  the  vicinity  of 
the  transverse  fissure. 

The  gall-bladder,  the  pear-shaped  receptacle  for  the  bile  attached  to 
the  under  side  of  the  liver  near  its  anterior  border,  possesses  strong  walls 
consisting  of  three  coats:  the  miccous,    the  muscular  and  the  Jibrous,   the 


Epithelium 


Blood-vessel 


Muscle 


Fig.  227. — Section  of  wall  of  gall-bladder,  showing  plicated  condition  of  mucous  membrane.    X  100. 


latter  supplemented  by  a  more  or  less  extensive  investment  of  peritoneum. 
The  mucous  coat,  covered  with  a  single  layer  of  columnar  epithelium  about 
5  p.  thick,  is  modelled  by  a  network  of  slightly  raised  ridges  that  mark  off 
irregular  polygonal  areas  some  5  mm.  in  diameter.  These  areas  are  often 
marked  by  minute  tubular  depressions  of  the  mucosa  which  have  been  mis- 
taken for  glands.  True  branched  mucous  glands  occur  in  the  neck  of  the 
gall-bladder,  but  are  so  few  in  other  parts  of  the  sac  as  to  be  practically 
wanting.  The  epithelial  cells  exhibit  a  cuticular  border  and  are  often  of 
the  goblet  type  and  concerned  in  producing  a  mucous  secretion.  The 
tunica  propria  contains  a  profusion  of  elastic  fibres  intermingled  with  the 
white  fibrous  tissue.  The  jnuscular  coat  is  composed  for  the  most  part  of 
circular  bundles  of  unstriped  fibres,  but  with  these  are  interwoven  longitudinal 


THE    PANCREAS.  183 

and  oblique  ones.  Outside  the  muscle  lies  a  d*tnse.JibroHS  coat  of  fibro-elastic 
connecti\e  tissue.  Where  invested  with  peritoneum,  the  latter  is  attached 
to  the  proper  wall  of  the  sac  by  a  layer  of  fat-laden  subserous  tissue. 

The  cystic  and  common  bile-ducts  possess  walls  that  in  structure 
correspond  with  the  hepatic  duct  above  described,  consisting  of  a  mucous 
tunic  strengthened  by  bundles  of  unstriped  muscle.  At  the  lower  end  of 
the  common  bile-duct,  the  circular  fibres  are  greatly  augmented  and  form  a 
sphincter-like  ring  around  the  orifice  of  the  tube  where  it  opens  into  the 
duodenal  ampulla.  The  bile,  the  secretion  of  the  liver,  contains  no  dis- 
tinctive cells,  numerous  minute  oil  drops  and  granular  masses  of  biliary 
pigment,  with  occasional  remains  of  the  epithelial  cells  lining  the  ducts, 
being  the  more  common  objects  encountered  when  the  fresh  fluid  is  exam- 
ined microscopicallv. 

The  blood-vessels  of  the  liver — the  functional  portal  vein,  the  nutrient 
hepatic  arterv  and  the  emergent  hepatic  veins — have  been  sufficiently  de- 
scribed. It  should  be  noted,  however,  that  the  blood  conveyed  to  the  organ 
by  the  hepatic  artery  is  destined  for  the  nutrition  of  the  interlobular  struct- 
ures, the  capsule  of  Glisson  and  the  walls  of  the  blood-vessels  and  of  the 
bile-ducts.  After  supplying  these  through  numerous  although  small  twigs, 
the  blood  is  collected  by  venous  radicles  and  emptied  either  into  interlobular 
branches  of  the  portal  vein  or  into  the  intralobular  capillary  network. 

The  lymphatics  of  the  liver  are  represented  within  the  lobules  by 
minute  Ivmph-spaces  between  the  blood-channels  and  the  liver-cells.  These 
spaces  drain  into  the  more  definite  lymphatic  paths  within  the  interlobular 
connecti\-e  tissue,  which  as  the  deep  lymphatics  surround  the  blood-vessels 
and  ducts  with  plexuses  that  condense  into  the  fifteen  or  more  trunks  emerg- 
ing at  the  trans\erse  fissure.  The  superficial  lymphatics,  very  numerous  and 
freely  communicating  with  the  deep  set,  arise  from  a  close-meshed  network 
of  lymph-channels  within  the  fibrous  capsule. 

The  nerves  of  the  liver,  from  the  solar  through  the  hepatic  plexus, 
consist  mostly  of  nonmedullated  fibres,  very  sparingly  intermingled  with 
meduUated  ones.  The  former  are  destined  chiefly  for  the  walls  of  the  blood- 
vessels and  of  the  larger  ducts,  which,  after  sending  filaments  to  the  capsule, 
they  follow  within  the  interlobular  tissue,  where  occasional  nerve-cells  are 
found  along  their  course.  Some  few  fibres,  probably  secretory  in  function, 
penetrate  the  lobules  to  end  between  the  liver-cells.  The  meagre  meduUated 
sensory  fibres  terminate  within  the  interlobular  connective  tissue. 

THE  PANXREAS. 

The  pancreas,  sometimes  called  the  abdominal  salivary  gland,  is  a  large 
tubo-alveolar  gland  that  lies  behind  the  stomach,  extending  from  the  loop  of 
the  duodenum  across  the  spine  and  left  kidney  often  as  far  as  the  spleen.  It 
is  conventionally  divided  into  the  head,  embraced  by  the  duodenum,  the  body 
and  the  tail.  The  interlobular  connective  tissue  is  unusually  abundant; 
hence  the  compartments  of  gland-tissue  are  loosely  united  and  the  entire 
organ  lacks  the  compactness  ordinarily  seen  in  large  glands.  While  agreeing 
in  its  general  structure  with  other  serous  glands,  as  the  parotid,  the  pancreas 
differs  in  certain  particulars.  The  most  important  of  these  are:  (a)  the 
tubular,  rather  than  saccular,  form  of  the  alveoli;  {b)  the  marked  differen- 
tiation of  a  granular  zone  in  the  cytoplasm  of  the  secreting  cells;  (<:)  the 
absence  of  specialized  intralobular  ducts;  and  (^)  the  presence  of  the  char- 
acteristic islands  of  Langerhans. 


1 84 


NORMAL    HISTOLOGY. 


The  chief  pancreatic  duct,  whose  walls  consist  of  a  single  layer  of 
unusually  tall  (12  ix)  columnar  epithelium  and  a  tunica  propria  of  compact 


Island  of 
Laugerhans 


Fig.  228.— Section  of  pancreas,  showing 


eneral  arrangement  of  lobules.     X  30. 


nbro-elastic  tissue,   gives  off   numerous   lateral  interlobular   branches,   also 
lined  by  a  simple  although  lower  epithelium.     The  canals  springing  from  the 


Alveolus 


-  Intralobular 
duct 


Interlobular 
duct 


% 


^■■^t^J  Vi 


t\'%igy^ 


_  Interlobular 
blood- 
vessels and 
nerve 


Fic.  229. — Section  of  pancreas,  showing  interlobular  tissue  with  vessels,  nerve  and  duct  and  surrounding 

tubular  alveoli.      •'  200. 

interlobular  ducts  enter  the  lobules,  where  they  are  lined  by  flattened  epithe- 
lial cells  ^3  //.  high)  and  correspond  to  intermediate  and  not  to  the  usual 
intralobular  ducts.      The  latter  being  wanting,  the  uncommonly  long  inter- 


THE    PANCREAS. 


i«5 


Fig.  230. — Portion  of  pancreas. 
treated  with  Golgi  method,  show- 
ing secretion-canaliciili  extend injj 
between  the  gland-cells.      ■   50. 


mediate  ducts  pass  directly  to  the  tubular  alveoli  into  many,  but  not  ail,  of 
which  their  attenuated  epithelium  protrudes  as  the  centroacinal  cells.  The 
relation  of  the  latter  to  the  secretory  elements  within  the  alveolus  is  such 
that  the  thinned-out  duct-cells  are  surrounded  externally  by  the  gland 
epithelium,  which  is  thus  excluded  from  direct 
contact  with  the  lumen  of  the  alveolus. 

The  tubular  alveoli,  often  tortuous  and 
sometimes  divided,  possess  a  well-defined  base- 
ment membrane,  against  which  lie  the  gland-cells. 
The  latter  are  usually  blunt  pyramidal  in  shape, 
with  a  length  of  about  12  //..  Their  cytoplasm 
exhibits  two  well  differentiated  zones,  an  inner 
granular  one,  next  the  lumen,  filled  with  highly 
refracting  zymogen  granules,  and  an  outer  clear 
one,  next  the  basement  membrane,  which  is  free 
from  granules  and  contains  the  spherical  nucleus. 
The  relative  width  of  these  zones  varies  with  the  functional  condition  of  the 
cells.  During  rest,  when  the  cells  are  stored  with  zymogen  particles,  the 
granular  zone  is  very  broad  and  the  outer  homogeneous  one  correspondinglv 
narrow.  With  discharge  of  the  pancreatic  secretion  during  digestion,  the 
granular  zone  diminishes  and  reaches  its  minimum,  almost  disappearing, 
when  the  gland  is  exhausted.  The  return  to  a  condition  of  rest  is  accom- 
panied by  the  formation  and  accumulation  of  a  new  store  of  zymogen  particles 
until  the  granular  zone  is  again  at  its  maximum.  Intercellular  sccrction- 
cmialiciili  are  present  in  all  alveoli  in  which  the  centroacinal  cells  exclude 
the  gland-cells  from  the  lumen.  They  extend  between  the  cells  almost,  but 
not  quite,  to  the  basement  membrane  (Fig.  230)  and  serve  to  convey  the 

secretion-products  through  the 
obstructing  central  cells  into 
the  lumen  of  the  alveolus. 

The  interalveolar  cell- 
areas,  or  islands  of  Lan- 
gerhans,  appear  as  small 
collections  of  modified  gland- 
tissue,  some  3  mm.  in  diameter, 
lying  between  the  tubular 
alveoli,  from  which  they  are 
separated  by  delicate  envelopes 
of  fibrous  tissue.  These  bodies 
consist  of  anastomosing  solid 
cords  or  trabeculae  of  small 
polyhedral  cells,  faintly  gran- 
ular and  without  zone  differ- 
entiation, separated  by  wide 
capillary  blood-vessels,  the 
whole  recalling  the  arrange- 
ment of  the  liver-substance. 
No  extension  of  or  connection  with  the  duct-system  of  the  organ  has  been 
demonstrated  within  these  areas,  secretion-canaliculi  likewise  being  wanting. 
In  view  of  their  isolation  from  the  surrounding  glandular  tissue  and  their 
intimate  relation  with  the  blood-channels,  the  interalveolar  cell-areas  are 
now  generally  believed  to  be  concerned  in  producing  some  special  substance 
that  passes  directly  into  the  circulation;  they  may  be  regarded,  therefore,  as 


.*^r. 


'n»- 


■  •-f. 


si     %    -'f 


Connective 
-tissue 
capsule 


?st 


«.%jCl  Blood-vessel 


Fig. 


231. — Section   of   pancreas,  showing 
of  Langerhans.     X  190. 


Modified 
cells 


Alveolar 
"cells 


details  of   island 


1 86 


NORMAL    HISTOLOGY. 


minute  multiple  organs  of  internal  secretion.  They  are  developed  from  the 
same  tissue  which  gi\-es  rise  to  the  ordinary  glandular  elements  of  the  pancreas 
and  are  constant  features  not  only  of  the  human  organ,  but  also  of  the 
•pancreas  of  a  wide  range  of  animals  representing  mammals,  birds,  reptiles 
and  amphibians.  Their  distribution  throughout  the  pancreas  is  by  no  means 
uniform,  since,  although  about  equally  numerous  in  the  head  and  adjacent 
part  of  the  body,  they  may  be  almost  double  in  number  towards  the  tail. 

The  pancreatic  duct,  or  duct  of  Wirsung^  empties  into  the  second 
part  of  the  duodenum,  usually  by  an  orifice  common  to  it  and  the  common 
bile-duct.  This  relation  is  explained  by  the  fact,  that  the  beginning  of  the 
later  pancreatic  duct  is  derived  from  an  outgrowth  of  the  wall  of  the  primary 


Fig.  232. — Section  of  injected  pancreas    showing  intralobular  capillar\  network;  also  vascular 
convolutions  of  islands  of  Langerhans.     X  50. 

liver-diverticulum  that  becomes  the  bile-duct,  both  excretory  canals  being 
thus  closely  connected  from  the  first.  The  walls  of  the  main  duct  and  its 
larger  branches  contain  numerous  small  tubular  mucous  glands  and  bundles 
of  unstriped  muscle.  At  its  lower  end,  the  circular  bundles  are  condensed 
and  augmented  into  a  sphincter-like  band  that  encircles  its  duodenal  orifice. 
The  blood-vessels  supplying  the  pancreas  are  distributed  to  the 
glandular  tissue  in  accordance  with  the  usual  plan  for  such  structures.  The 
positions  of  the  interalveolar  cell-areas,  however,  are  indicated  in  minutely 
injected  organs  by  corresponding  areas  in  which  the  capillary  network  is 
exceptionally  dense  (Fig.  232).  The  lymphatics  are  represented  by 
definite  channels  accompanying  the  interlobular  blood-vessels  to  which  the 
intralobular  lymph-spaces  are  tributary.  The  nerves  are  composed  chiefly 
of  nonmedullated  sympathetic  fibres  distributed  to  the  walls  of  the  blood- 
vessels and  of  the  larger  ducts,  some  passing  to  the  alveoli.  Ganglion-cells, 
scattered  or  grouped,  lie  along  the  interlobular  trunks. 


THE  ORGANS   OF   RESPIRATION. 

The  respiraton'  tract  proper,  that  is  excluding  the  nasal  fossae  (through 
which  the  air  passes  when  the  mouth  is  closed)  and  the  pharynx,  includes 
the  organs  concerned  in  effecting  the  interchange  of  the  gases  between  the 
blood  "and  the  inspired  air  and,  in  addition,  the  production  of  voice.  It 
comprises  the  larynx,  the  trachea  and  its  subdi\isions — the  bro7ichi,  and  the 
hoigs,  together  with  the  serous  membranes,  the  pleura',  which  surround 
the"  lungs  and  line  the  spaces  containing  them.  The  respiratory  tract  is 
developed  as  a  ventral  outgrowth  from  the  primitive  gut-tube  and  is  lined,  • 
therefore,  by  entodermic  epithelium,  all  other  parts  of  the  organs  being 
derived  from  the  mesoderm. 


.^— Hyoid  cormi 

L Thyro-hyoid 

ligament 

^  V—  Thyroid  comu 


Cuneiform 
cartilage 
Corniculate 
cartilage 


False  vocal  fold 
Vocal  fold 


THE  LARYNX. 

The  larvnx  consists  of  a  fibro-cartilaginous  framework  lined  with  mucous 
membrane   and  surrounded  by  muscles.      By  the  action  of  the  latter  the 
relative  position  of  the  cartilages  is  modified,  thereby  affecting  the  approxi- 
mation and    tension  of   two 
folds  of  mucous  membrane, 
known  as  the  vocal    cords, 
that  cover  the  free  edges  of 
fibro-elastic  membranes  and 
bound     the     cleft     through 
which  the  air  passes  to  and 
from  the  windpipe. 

The  cartilages  of  the 
larynx  include  three  single 
ones:  the  cricoid,  the  thyroid 
and  the  epiglottis ;  and  three 
paired  ones:  the  arytenoid, 
the  cornicular  and  the  cunei- 
form, the  last  being  small  and 
sometimes  wanting.  Other 
minute  masses  of  cartilage 
will  be  noted  in  connection 
with  the  structures  in  which 
they  occur.  In  structure  the 
laryngeal  cartilages  corre- 
spond to  the  hyaUne  variety,  with  the  exception  of  the  epiglottis,  the  tip  and 
vocal  process  of  the  arytenoid,  the  cuneiform,  the  corniculate  and  often. 
but  not  always,  the  median  part  of  the  thyroid,  which  exceptions  consist  of 
elastic  cartilage.  Beginning  about  the  twentieth  year,  more  or  less  extensive 
ossification  of  the  cartilages,  especially  the  thyroid  and  cricoid,  occurs  as  a 
normal  change.  The  arytenoid  cartilages  are  less  affected  and  the  median 
part  of  the  thyroid  is  said  to  remain  unchanged  in  women. 

The  mucous  membrane  lining  the  larynx  is  a  prolongation  of  that 
of  the  pharynx,  and  consists  of  the  epithelium  and  tunica  propria,  with  the 
underlying  submucous  tissue  by  which  the  mucosa  is  attached  to  the  sur- 
rounding framework.      The  epithelium  is,  for  the  most  part,  stratified  ciliated 

187 


Epiglottis 


Hvoid  bone 


Thyro-thyroid 
membrane 
Mass  of  fat 


Ventricle 
Thyroid  cartilage 

Cricoid  cartilage 

Trachea 


Fig.  233.— Median   sagittal  section  of  larynx  ;    right  half,  seen 
from  within.     ,<  %. 


i88  .  NORMAL   HISTOLOGY. 

columnar,  except  over  the  laryngeal  surface  of  the  epiglottis,  the  anterior 
aspect  of  the  arytenoid  cartilages  and  the  true  vocal  cords,  where  the  epi- 
thelium is  of  the  stratified  squamous  variety.  In  this  respect,  however,  the 
mucosa  of  the  upper  half  of  the  larynx  presents  many  individual  variations, 
since  patches  of  squamous  epithelium  are  often  observed  within  the  more 
general  lining  of  columnar  cells.  Small  scattered  taste-buds  are- often  seen 
in  the  epithelium  covering  the  epiglottis.  The  tunica  propria  consists  of 
closely  packed  bundles  of  fibrous  tissue,  with  an  abundance  of  elastic  fibres. 
It  contains  many  lymphoid  cells  which  in  certain  locations,  as  over  the  epi- 
glottis and  especially  in  the  ventricle  of  the  lar3-nx  (the  lateral  diverticulum 


Glands 


Epiglottis 


Fat 


False  vocal  cord 


Lymphoid  tissue 
True  vocal  cord 


Thyro-aryteuoid 
muscle 


Thyroid  cartilage 


Cricoid  cartilage 

Fig.  234. — Frontal  section  of  larynx  passing  through  epiglottis,  vocal  folds  and  ventricle;  the  plane  of 
section  is  at  right-angles  to  that  of  preceding  figure.     A  2%. 

between  the  false  and  true  vocal  cords),  are  aggregated  into  distinct  although 
small  lymph-nodules  (Fig.  234). 

The  submucous  layer,  composed  of  loosely  disposed  bundles  of  fibre- 
elastic  tissue,  varies  in  amount  in  different  parts  of  the  larynx,  with  corre- 
sponding modifications  in  the  intimacy  of  attachment  of  the  mucous  membrane 
to  the  surrounding  structures.  Thus,  it  is  meagre  over  the  free  part  and 
laryngeal  surface  of  the  epiglottis,  the  arytenoid  and  lower  part  of  the  cricoid 
cartilages,  in  which  positions  the  mucosa  is  closely  attached.  Over  the  true 
vocal  cords  the  submucosa  is  practically  wanting.  On  the  other  hand, 
in  the  aryepiglottic  folds  (which  bound  laterally  the  superior  laryngeal  orifice) 
and  in  the  ventricle  it  is  abundant,  with  consequent  mobility  of  the  mucosa. 
The  submucous  layer  contains  many  groups  of  small  mixed  mucous  tubo- 
alveolar  glajids.     They  occupy  pits  in   the   cartilage  of  the  epiglottis,    and 


THE    LARYNX. 


189 


are  numerous  and  relatively  large  over  the  false  vocal  cords  {pliccB  ventricu- 
lares)  and  plentiful  in  the  ventricles.  They  do  not  occur  on  the  upper 
surface  of  the  true  vocal  cords  within  3-4  mm.  of  their  free  margins,  but 
beneath  the  latter  the  glands  form  almost  a  continuous  layer. 

The  vocal  cords,  more  appropriately  called  \\\& pliccc  vocalcs,  are  two 
duplicatures  of  mucous  membrane  which  cover  the  free  median  margins  of 
the  lateral  crico-thyroid  membranes.  This  part  of  the  membrane,  often 
designated  the  thyro-arytenoid  ligament,  is  attached  to  the  thyroid  cartilage 
in  front  and   to  the  vocal  process  of  the  mobile  arytenoid  cartilage  behind, 


Longitudinal  muscle 
Submucous  layer 


Epithelium  of 
oesophagus 


Circular  muscle 


Muscle 


-  Mucous 
membrane 


V-  Trachea 


%• 


-'fit*: 


My' 


Glands 


Cartilage 


Fig.  235. — Transverse  section  of  trachea  and  (esophagus  of  child.     X  15. 

and  is  directly  influenced  by  the  contractions  of  the  thyro-arytenoid  muscle, 
which  lies  against  the  membrane  externally  and  inserts  many  of  its  fibres 
directly  into  tlie  fibrous  band.  The  submucous  tissue  being  wanting  over 
the  vocal  cord,  the  here  thin  mucous  membrane  is  intimately  attached  to  the 
underlying  fibrous  stratum  of  the  thyro-arytenoid  ligament,  thus  insuring 
accurate  response  to  the  changes  induced  by  muscular  action.  Small  masses 
of  elastic  cartilage,  from  2—3  mm,  long,  are  occasionally  found  in  the  anterior 
ends  of  the  vocal  cords;  smaller  pieces  of  similar  tissue  are  quite  common 
in  the  ventricular  plicce. 

The  numerous  blood-vessels  supplying  the  larynx  are  distributed 
chiefly  to  the  mucous  membrane,  in  which  the  main  capillary  network  lies 
close  beneath  the  epithelium.  Other  branches  are  given  ofT  within  the  sub- 
mucous layer  and  provide  the  capillary  supply  for  the  numerous  glands. 
The  lymphatics  are  well  represented  throughout  the  greater  part  of  the 
laryngeal  mucous  membrane,  especially  in  the  region  of  the  ventricle  and 
false  vocal  folds.  Over  the  true  cords,  however,  they  are  very  feebly  devel- 
oped, but  below  them  the  lymphatics  are  again  numerous.  Where  abundant, 
the  lymph-channels  are  present  within  the  mucosa  as  a  superficial  and  a  deeper 
network,  which  communicate  and  pass  to  the  cervical  lymph-nodes. 


I90  NORMAL    HISTOLOGY. 

The  nerves  supplying  the  larynx  are  chiefly  from  the  vagi,  intermingled 
with  fibres  from  the  sympathetic.  They  consist,  therefore,  of  meduUated  and 
nonmedullated  fibres,  groups  of  ganglion-cells  occurring  along  the  course  of 
the  sympathetic  fibres.  The  latter  are  destined  for  the  blood-vessels  and 
glandular  tissue.  The  muscles  being  of  the  striated  variety  are  supplied  by 
fibres  bearing  motor  end-plates.  The  sensory  fibres  are  distributed  princi- 
pally to  the  mucous  membrane,  in  which  they  form  plexuses.  From  the  latter 
nonmedullated  fibres  pass  towards  the  free  surface  to  terminate  either  in  sub- 
epithelial end-arborizations,  bearing  enlargements  or  end-bulbs,  or  in  intra- 
epithelial filaments  ending  free  between  the  cells.  Special  end-organs 
have  been  described  as  existing  within  the  true  vocal  cords. 

THE  TRACHEA  AND  BRONCHL 

Beginning  at  the  lower  border  of  the  cricoid  cartilage,  the  trachea  or 
windpipe  extends  into  the  thorax,  a  distance  of  some  10-12  cm.,  and  divides 
into  two  bronchi,  one  proceeding  downwards  and  outwards  into  each  lung. 

,-«#T:;^ Epithelium 

_,i^^^^^  Tunica  propria 

^-'  Tracheal 

2^  glands 


i" 


-Submucous 
layer 


-Cartilage 


Perichondrium 


-Fibrous  tunic 


Fig.  236. — Transverse  section  of  trachea,  showing  general  arrangement  of  the  coats.    X  80. 

Until  the  latter  is  reached,  the  walls  of  the  air-tubes  are  constant  in  structure 
and  consist  essentially  of  a  fibro-cartilaginous  framework,  lined  with  mucous 
membrane  and  covered  externally  with  areolar  tissue. 

The  cartilage   is  represented  by  a  series  of  from  sixteen  to  twenty 
C-shaped  pieces,  of  the  hyaline  variety  and  from  2-5  mm.  in  width,  which 


THE   LUNGS.  191 

are  united  with  one  another  by  a  fibro-elastic  sheath  continuous  with  the 
perichrondrium  of  the  segments.  This  membrane  Ukewise  connects  the 
ends  of  the  "rings,"  as  the  pieces  are  called,  and  thus  completes  the  tube 
behind,  where  otherwise  the  framework  of  the  posterior  wall  of  the  trachea 
and  bronchi  would  be  deficient.  In  order  to  maintain  the  proper  tonicity  of 
the  fibro-cartilaginous  tube,  especially  of  its  membranous  portion,  bundles  of 
unstriped  muscle,  the  trachealis  vmscle,  lie  between  the  ends  of  the  cartilages. 
In  the  main  these  bundles  are  disposed  circularl\-,  connecting  the  ends  of  the 
rings,  but  some  run  longitudinally. 

The  mucous  membrane,  smooth  and  attached  with  considerable 
firmness  to  the  cartilages  by  the  submucous  tissue,  but  looser  and  thrown 
into  longitudinal  folds  over  the  posterior  wall,  is  clothed  with  stratified  ciliated 
columnar  epithelium.  ^lany  of  the  surface  cells  contain  mucus  and  are  of  the 
goblet  \ariety.  The  tunica  propria  is  rich  in  elastic  fibres  and  contains 
numerous  lymphoid  cells,  which  are  so  abundant  in  places,  particularly 
around  the  openings  of  the  tracheal  glands,  as  to  suggest  lymph-nodules. 
The  submucous  layer,  composed  of  fibro-elastic  tissue,  lodges,  in  addition 
to  the  larger  blood-vessels  and  lymphatics,  the  tracheo-bronchial  gla7ids. 
These  occur  as  considerable  masses  (Fig.  236)  and  belong  to  the  mixed 
mucous  tubo-alveolar  type.  Their  ducts  pierce  the  tunica  propria  and  open 
on  the  free  surface  by  minute  funnel-like  depressions  in  the  epithelium.  The 
blood-vessels,  lymphatics  and  nerves  follow  essentially  the  same  plan  of  dis- 
tribution as  described  in  connection  with  the  larynx. 

THE   LUNGS. 

In  their  mode  of  development  and  architecture,  the  lungs  resemble 
compound  alveolar  or  saccular  glands,  the  repeatedly  subdividing  air-tubes 
Cthe  bronchioles)  representing  the  duct-system  of  a  gland  and  the  ultimate 
compartments  of  the  respiratory  tissue  (the 
alveoli)  corresponding  to  the  glandular 
alveoli.  Instead  of  being  almost  filled  with 
secreting  cells,  however,  after  birth  the  pul- 
monary alveoli  are  distended  with  air  and 
the  cells  reduced  in  thickness  to  endothelium- 
like  plates. 

The  Lobule  and  Lung-Units. — The 
surface  of  the  lung  (Fig.  237)  is  marked 
with  small  polygonal  areas,  10-25  mm.  in 
diameter,  which  are  defined  by  lines  of  con- 
necti\e  tissue,  often  darkened  by  pigment. 
These  areas  are  the  bases  of  pyramidal 
masses  of  pulmonary  tissue,  the  lobules, 
each  of  which  is  entered  by  and  surrounds 
a  minute    air-tube,   iyitralobidar   bronchiole, 

from  .5-1  mm.  in  diameter,   accompanied  by       showing  "polygonal  areas,  correspondiirg 
1  ,  r     .  1  1  ^  T^i  to  the  lobules,  mapped  out  by  pigment 

a   branch   of   the    pulmonary  artery.      The     within  the  connective  tissue, 
bronchiole  enters  the  lobule  near,  but  not  quite 

at,  its  apex  and  divides  into  two  a  little  above  the  middle  of  the  lobule,  having 
previously  given  off  two  or  three  collateral  branches  to  its  upper  part.  In 
the  third  quarter  of  the  lobule,  the  two  branches  subdivide  in  a  plane  at 
right-angles  to  the  preceding  splitting.  Such  division  is  repeated  in  three 
or  four  successive  bifurcations,  a  varying  number  of  collaterals  being  given 


Fig.    237. — Externa!    surface    of    lung, 


ig2 


NORMAL   HISTOLOGY. 


Fig.  23S. — Diagram  illustratins?  relations  of 
terminal  divisions  of  air-tubes.  B,  bronchiole 
ending  in  terminal  bronchi  (  TB)\  latter  divide 
into  atria  (A),  each  of  which  communicates 
with  several  air-sacs  {s)  into  which  open  the 
alveoli  (a)\  PA,  branch  of  pulmonary  artery 
following  bronchiole  ;  PV,  pulmonary  vein  at 
periphery  of  lung-unit.     (  Miller.) 


off  in  addition.  Although  the  number  of  branches  of  the  air-tubules  is 
much  increased  in  the  third  quarter,  it  is  in  the  last  one  and  towards  the 
periphery  of  the  lobule  throughout,  that  the  tubules  break  up  into  a 
profusion  (50-100)  of  truly  terminal  bronchioles.      Each  terminal  bronchiole 

communicates  by  its  slightly  dilated 
distal  extremity  with  from  three  to  six 
spherical  cavities,  the  atria,  each  of 
which,  in  turn,  communicates  with  a 
group  of  larger  irregular  cavities,  the 
alveolar  sacs,  into  which  directly  open 
the  ultimate  air-spaces,  the  pulmonary 
alveoli.  The  latter  open  not  only  into 
the  alveolar  sacs  but  also  into  the  atria 
and  even  the  distal  part  of  the  terminal 
bronchiole,  which  is  beset  with  scattered 
alveoli.  The  mass  of  pulmonary  tissue 
connected  with  each  terminal  bronchiole, 
including  the  air-spaces  and  accompany- 
ing blood-vessels  and  nerves,  constitutes 
a  lung-unit,  by  the  aggregation  of  which 
the  lobule  is  formed.  The  lobules  are 
separated  by  distinct  tracts  of  interlobular 
connective  tissue,  in  which  the  air-tubes 
and  accompanying  blood-vessels  course 
until  they  enter  the  lobules. 

The  Bronchioles. —  After  the 
bronchus  begins  to  give  off  branches 
within  the  lung,  the  cartilage-rings  gradually  decrease  in  size  and  thickness 
until  replaced  by  irregular  angular  plates,  which  appear  at  increasingly  longer 
intervals  until  they  finally  cease,  cartilage  being  seldom  present  in  bronchioles 
of  less  than  i  mm.  in  diameter.  As  the  cartilage  tends  to  disappear,  the 
unstriped  muscle  broadens  into  a 
continuous  layer,  which,  in  turn, 
becomes  thinner  as  the  air-tube 
diminishes  and  extends  only  as  far 
as  the  terminal  bronchioles.  The 
muscle  is  arranged  as  a  sphincter- 
like band  around  the  openings 
by  which  the  terminal  bronchioles 
communicate  with  the  atria. 

The  walls  of  bronchioles  of 
medium  size  (2-3  mm. )  consist 
of  three  coats,  which  from  with- 
out in  are:  (i)  an  ^.^XoxrvA fibrous 
tunic,  composed  of  fibro-elas'tic 
tissue,  which  encloses  the  cartilage 
(often  elastic  in  type)  and  accom- 
panying blood-vessels  and  blends 
with  the  adjoining  lung-tissue;  (2) 
a  thin  layer  of  unstriped  muscle,  sometimes  incomplete  and  composed  of 
circularly  disposed  bundles;  and  (3)  the  mucoiis  membrane, ^  thrown  into 
longitudinal  folds  and  consisting  of  ciliated  columnar  epithelium,  with  nu- 
merous goblet-cells,  and  a  tunica  propria  made  up  chiefly  of  meshes  of  elastic 


Fig.  239. — Corrosion-preparation  of  lung,  showing 
lung-units;  a,  minute  bronchus  ending  in  terminal 
bronchi  \b);  c,  atria;  rf,  air-sacs;  e,  alveoli.    X  8. 


THE   LUNGS. 


193 


fibres  and  intervening-  lymphoid  cells.     As  in  the  trachea  and  the  bronchi, 
the  current  produced  by  the  cilia  is  directed  centrally,  and  thus  tends  to 


Bronchiole 


Bronchiole 


Mucous 
meiibrane  ^*?7~~^^ 


Cartilage 


Tunica  • 
propria  ■ 

-Epithelium 
-Goblet-cell 


Fig.  240.— Section  of  lung,  showing  small  air-tubes  and  branch  of  pulmonary  artery.      <  35. 

carry  mucus  and  particles  of  dust  from  the  smaller  tubes  towards  the  bronchi 
and  trachea.  Mucous  glands,  similar  to  those  of  the  trachea,  are  present  in 
decreasing  number  and  size 

until  the  bronchiole  attains  /     ^  ^^    ■■     ^  "t?5g.=^ 

a  diameter  of  i  mm.,  after 
which  they  usually  disap- 
pear. Their  chief  location 
is  outside  the  muscle,  which 
is  pierced  by  the  ducts  on 
their  journey  to  gain  the 
free  surface  where  they 
open  in  minute  depressions 
within  the  epithelium.  In 
addition  to  the  lymphoid 
cells  diffused  through  the 
mucosa,  more  definite  ag- 
gregations occur  as  minute 
Ivmph-nodules  along  the 
bronchi,  the  points  of  bifur- 
cation being  tlieir  favorite 
seats.  The  epithelium  lin- 
ing the  air-tubes  retains  the 
ciliated  columnar  type  as 
far  as  the  smaller  bronchi- 
oles. Within  the  latter 
the  ciliated  cells  are  replaced  by  simple  columnar  elements,  which,  in  turn, 
give  way  to  low  cuboidal  cells  within  the  proximal  part  of  the  terminal 

13 


Muscle 


Fibrous  tissue 
Mveolar  wall 


Cartilage 


Fig.  241. — Portion  of  wall  of  small  bronchus.    X  180. 


194 


NORMAL   HISTOLOGY. 


bronchioles.  Towards  the  distal  ends  of  the  latter  partial  transition  into 
patches  of  simple  squamous  epithelium  occurs,  these  tubules  containing, 
therefore,  both  cuboidal  and  plate-like  cells. 

The  Air-Spaces. — The  walls  of  the  air-spaces — the  atria,  the  alveolar 
sacs  and  the  pulmonary  alveoli — have  essentially  the  same  structure  and 
consist  of  a  delicate  fibro-elastic  framework  supporting  the  blood-vessels 
and  the  epithelium.  The  latter,  the  respiratory  epithelium,  is  made  up  of  a 
single  layer  of  polygonal  plates,  mostly  without  nuclei,  and  includes  groups 
of  large  and  small  cells.  The  number  of  the  smaller  cells,  as  seen  in  silvered 
preparations  of  adult  lung  (Fig.  243),  progressively  decreases  towards  the 
alveoli,  in  which  they  are  reduced  to  small  groups  or  isolated  elements  sur- 
rounded by  the  larger  plates.  In  the  foetus  and  in  the  still-born  child,  the 
alveoli  are  lined  entirely  by  low  cuboidal   cells;  after  inflation  of  the  lung- 


Air-sac 


Air-sacs 


Passage  from 
atrium  into  air-sac^ 


Alveolu: 

Terminal  bronchus 

Pulmonary  arten' 
Bronchiole 


Atrium 


Alveolus 


Fig.  242.— Section  of  lung,  showing  relations  of  terminal  divisions  of  air-tubes.    X  50- 

tissue  has  been  completed,  the  cuboidal  cells  become  expanded  into  the 
small  plates.  The  larger  plates  arise  by  the  subsequent  fusion  of  several  of 
the  small  ones,  groups  of  the  latter,  which  retain  their  independence,  appear- 
ing, in  decreasing  numbers  as  age  advances,  as  the  islands  of  one  or  more 
small  cells. 

The  framework  of  the  pulmonary  alveoli  is  almost  exclusively  elastic 
fibres  that  are  condensed  into  rings  around  the  openings  or  bases  of  the 
alveoli  and  elsewhere  enclose  the  spaces  with  elastic  networks.  Where  these 
rings  come  into  contact  and  fuse,  the  alveoli  are  separated  by  partitions  of 
some  thickness;  beyond  these  septa  the  walls  between  the  adjoining  air- 
spaces are  very  thin  and  include  the  two  layers  of  epithelial  plates  and  the 
dense  capillary  network  supported  by  the  elastic  reticulum.  Owing  to  the 
elastic  character  of  their  walls,  the  alveoli  expand  during  inspiration  to  two 
or  three  times  their  usual  diameter  (.  i-.  3  mm.  ) ,  the  lining  epithelial  plates  and 
the  blood-vessels  stretching  to  the  necessary  degree.      The  capillary  netzvorks 


THE   LUNGS. 


195 


surrounding  the  alveoli  are  in  many  places  common  to  the  opposed  spaces 
beitinging-  to  the  same  unit.  These  networks,  the  terminations  of  the  pul- 
monary artery  and  the  beginnings  of  the  pulmonary  veins,  possess  exceed- 
ingly small  meshes,  the  distance  between  the  capillaries  often  being  less  than 
the  diameter  of  the  vessels.  The  latter,  not  confined  to  one  plane  but 
sinuous,  are  excluded  from  the  interior  of  the  alveoli  by  practically  only  the 
thin  layer  of  respiratory  epithelium,  an  arrangement  manifestly  advantageous 
in  effecting  the  interchange  between  the  carbon  dioxide  of  the  venous  blood 
and  the  oxygen  of  the  inspired  air. 

Although  preformed  openings  or  stomata  do  not  exist  between  the 
alveolar  epithelial  elements,  particles  of  foreign  materials  pass  through 
the  wall  of  the  air-spaces  and  eventually  into  the  interlobular  connective 


Ov 


^ 


Epithelium 
lining  al 
veoli 


v^O 


Fig.  243. — Section  of  lung,  showing  alveolar  epithelium  and  collections  of  colored  particles  in  connective 

tissue.    X  140. 

tissue,  where  they  accumulate  as  the  more  or  less  conspicuous  collections  of 
pigment  which  aid  in  defining  the  outlines  of  the  lobules.  A  certain  amount 
of  such  pigmentation  is  always  found  in  adult  lungs  and  may  be  regarded  as 
normal;  when,  however,  individuals  are  subjected  to  an  atmosphere  unduly 
laden  with  colored  particles,  as  coal  dust,  the  pulmonary  tissues  may  be  so 
filled  with  pigment  as  to  be  in  places  almost  black.  It  is  probable  that  the 
migratory  leucocytes  are  an  important  means  of  transporting  the  colored 
particles  from  the  alveoli  into  the  connective  tissues. 

The  blood-vessels  of  the  lung,  as  those  of  the  liver,  include  two  sets, 
one  for  the  function  of  the  organ,  the  other  for  the  nutrition  of  its  tissues. 
The  former  are  the  branches  of  the  pulmonary  artery  and  veins;  the  latter 
are  the  bronchial  vessels.  The  nutrient  arteries  arise  from  the  aorta,  not 
directly  from  the  heart.  The  branches  of  the  pulmonary  artery  follow 
closely  the  ramifications  of  the  air-tubes,  entering  the  lobules  near  their 
apices,  along  with  the  intralobular  bronchioles,  and  finally  breaking  up  into 
the  close  capillary  networks  in  the  walls  of  the  alveoli.      From  these  networks 


196 


NORMAL    HISTOLOGY. 


Capillary  net  _ 
work  over      v!?^^ 
alveolus 


Alveolus 


arise  the  radicles  of  l\\&p2il))i07ia?y  veins  which  carry  away  the  oxygen-renewed 
blood.  These  vessels,  however,  do  not  immediately  join  the  arteries,  but, 
running  first  on  the  outside  of  the  lung-units,  unite  with  others  and  then 
emerge  at  the  periphery  of  the  lobules  and  run  in  the  interlobular  connective 
tissue,  later  meeting  the  interlobular  parts  of  the  arteries  and  bronchi  which 
they  thence  accompany  to  the  hilum  of  the  lung.  At  the  surface,  where 
the  pulmonary  tissue  is  in  contact  with  the  overlying  serous  membrane, 
twigs  from  the  pulmonary  artery  communicate  with  the  pleural  capillaries. 
The  bronchial  aj'teries,  the  nutrient  vessels  of  the  lungs,  supply  the  walls  of 
the  air-tubes  as  far  as  the  terminal  bronchioles,  as  well  as  the  walls  of  the 
branches  of  the  pulmonary  artery  and  veins,  the  bronchial  lymph-nodes  and 
the  visceral  pleura.  Within  the  walls  of  the  bronchial  tubes  they  form  a 
deeper  capillary  network  for  the  muscle  and  glands  and  a  superficial  one  for 

the  tunica  propria.  The 
bronchial  veins  are  trib- 
utary for  the  most  part 
to  the  pulmonary  veins; 
to  a  small  extent,  how- 
ever, blood  passes  from 
them  into  the  azygos 
system.  Both  the  bron- 
chial arteries  and  veins 
comm.unicate  with  the 
pulmonary  vessels  at 
many  points. 

The  lymphatics 
include  a  superficial  net- 
work, well  developed 
and  beneath  the  pleura, 
and  a  deep  interlobu- 
lar plexus  surrounding 
the  bronchi.  The  deep 
ones  probably  begin  as 
lymph-spaces  distal  to 
the  terminal  bronchi- 
oles, around  which  tu- 
bules definite  lymphatics  first  appear.  The  superficial  vessels  are  con- 
nected with  small  uncertain  subserous  lymph-nodes,  subsequently  joining 
the  interlobular  trunks,  which  ultimately  are  efferent  to  the  larger  nodes 
situated  in  the  hilum  and  roots  of  the  lungs.  The  pulmonary  lymph- 
nodes  are  deeply  pigmented  owing  to  the  accumulation  of  inspired  colored 
particles.  Where  cartilage  exists,  the  plates  are  enclosed  by  double  net- 
works of  lymphatics,  the  inner  one  lying  within  the  submucosa. 

The  nerves  of  the  lungs,  from  the  vagi  and  sympathetics,  are  numerous 
and  include  both  medullated  and  nonmeduUated  fibres.  The  latter  are 
associated  with  minute  groups  of  ganglion-cells  along  their  course  and  are 
destined  chiefly  for  the  walls  of  the  blood-vessels  and  of  the  air-tubes,  some 
fibres  finding  their  way  into  the  interalveolar  septa.  Free  terminal  filaments 
within  the  tunica  propria  and  between  the  epithelial  cells  are  described  as 
sensory  endings  in  the  mucous  membrane  of  the  air-tubes. 

The  Pleurae. — The  pleurae,  the  serous  membranes  lining  the  cavities 
containing  the  lungs  and  covering  the  latter  except  at  the  roots,  where  enter 
the  bronchi  and  the  blood-vessels,  in  structure  closely  resemble  other  serous 


Branch  of  pulrrjonar^.  \ 


'^S 


Fig.  244. — Section  of  injected  and  inflated  lung.    X 


THE   THYROID    BODY. 


197 


membranes.  The  visceral  pleura  consists  of  a  stroma-layer,  composed  of 
fibrous  tissue  intermingled  with  an  abundance  of  elastic  fibres,  and  a  single 
surface  layer  of  mcsothelial  plates.  The  existence  of  definite  openings  or 
stomata  between  these  cells  is  doubtful. 
The  parietal  pleura  possesses  a  like  struct- 
ure, but  is  less  rich  in  elastic  fibres.  The 
subserous  layer  is  scanty  over  the  lung, 
where  it  is  continuous  with  the  interlobular 
connective  tissue;  over  the  mediastinum  it 
is  firm  and  dense  and  on  the  costal  wall 
acquires  the  character  of  a  fascia,  which  is 
particularly  dense  beneath  the  apical  pleura. 
The  blood-vessels  supplying  the  vis- 
ceral pleura  are  derivations  of  the  pulmonary 
trunks;  those  of  the  parietal  pleura  are 
from  various  adjacent  systemic  branches. 
In  neither  case  is  the  ultimate  distribution 
a  generous  one,  the  twigs  being  small  and 
the  capillaries  comparatively  few.  The 
lymphatics  are  most  abundant  over  the 
lungs  and  the  intercostal  spaces,  where  they 
form  meshworks  within  the  stroma  and 
the  subserous  tissue.  The  nerves  of  the 
visceral  pleura  include  fibres  from  the  vagi 
and  sympathetics  by  way  of  the  pulmonary 
plexuses.  Those  of  the  parietal  pleura  re- 
ceive fibres  from  the  intercostal  and  phrenic 
nerves  and,  additionally,  some  from  the  vagi 
and  the   sympathetics.      Many  of   the   sensory 


Interalveolar 

wall 


Endothelium 
or  free  surface 


iimective-tissue 
btroma  of  pleura 


Fig.  245. — Section  through  free  ed^e  of 
lung,  showing  visceral  pleura.     X  150. 


fibres  are  connected  with 
special  end-organs,  as  the  Pacinian  and  Golgi-Mazzonian  corpuscles,  while 
others  terminate  in  free  varicose  endings. 


Solely  as  a  matter  of  convenience,  in  view  of  their  contiguity,  the 
Thyroid,  Parathyroid  and  Thymus  Bodies  may  be  described  in  connection 
with  the  respiratory  tract.  It  must  be  clearly  understood,  however,  that 
these  organs  have  neither  morphological  nor  functional  relations  with  the 
organs  of  respiration;  they  are,  probably,  to  be  regarded  as  accessory  organs 
of  nutrition. 

THE   THYROID    BODY. 

The  thyroid  body  is  developed  from  three  rudiments,  an  unpaired 
median  and  two  lateral.  The  median  rudiment  or  anlage  is  an  entodermic 
epithelial  outgrowth  from  the  anterior  wall  of  the  primitive  pharynx,  in  the 
region  of  the  second  visceral  arch  and  in  close  relation  with  the  posterior 
part  of  the  tongue.  The  position  of  this  outgrowth  is  later  indicated  by  a 
depression  on  the  tongue,  the  foramen  caecum,  just  behind  the  apex  of  the 
row  of  circumvallate  papillae.  The  lateral  rudiments  appear,  one  on  each 
side,  as  epithelial  outgrowths  from  the  ventral  wall  of  the  fourth  pharyngeal 
furrows.  The  three  rudiments  grow  ventrally  and  subsequently  join  to  form 
the  definitive  thyroid  surrounding  the  respiratory  tube.  The  histogenesis 
of  the  organ  includes:  (a)  numerous  cylindrical  epithelial  cords  from  which 
grow  out  lateral  branches;   i h)  fusion  of  these  cords  into  a  network  whose 


NORMAL    HISTOLOGY. 


meshes  are  filled  with  vascular  mesodermic  tissue;  (c)  severance  of  the 
epithelial  reticulum  into  masses  corresponding  to  the  later  follicles;  (d)  the 
appearance  within  these  masses  of  lumina,  around  which  the  cells  become 
arranged  as  the  epithelial  lining  of  the  compartments  subsequently  containing 
the  characteristic  colloid  substance.  The  thyroid  body  agrees  with  the 
parathyroids  and  the  thymus  in  arising  from  the  walls  of  the  primitive 
pharynx  and  in  deviating  during  its  later  development  from  its  original 
likeness  to  a  typical  gland. 

The  thyroid  body  is  situated  in  the  neck,  in  front  and  at  the  sides  of 
the  upper  end  of  the  trachea,  and  consists  of  two  lateral  lobes  connected  by 
a  narrow  strip,  the  isthmus.      Although  during  its  early  development  cor- 


Colloid 

'"i^ —  listending^ 
follicle 


Interlobular 
septum 


Fig.  246. — Section  of  thyroid  body,  showing  follicles  in  various  degrees  of  distention.     X  100. 

responding  in  principle  with  compound  alveolar  glands,  the  fully  formed 
thyroid  body  possesses  no  excretory  ducts  and  varies  in  the  details  of  its 
terminal  compartments.  The  fibro-elastic  capsule  investing  the  organ  gives 
ofT  septa  which  subdivide  the  lobes  into  a  number  of  tracts,  each  composed 
of  smaller  masses,  the  primary  lobules,  separated  by  thin  partitions  of  con- 
nective tissue.  These  subdivisions  (.5-1  mm.  in  diameter)  contain  a  variable 
but  usually  large  number  of  follicles,  which  correspond  to  the  alveoli  of 
ordinary  glands  and  are  supported  by  a  highly  vascular  fibro-elastic  frame- 
work. 

The  follicles,  ellipsoidal  or  cylindrical  sacs,  vary  greatly  in  size  (50- 
200  //),  depending  upon  the  amount  of  the  contained  secretion  and  disten- 
tion. They  are  lined  by  a  single  layer  of  fairly  regular  epithelial  cells, 
usually  cuboidal,  although  they  may  approach  the  columnar  or  flattened 
type.  Their  spherical  nuclei  are  surrounded  by  clear  cytoplasm,  which 
often  contains  granules  of  a  fatty  nature.  The  epithelial  cells  are  the  source 
of  the  peculiar  soft  gelatinous  material,  the  colloid  substance,  that  fills  and 
distends  to  a  variable  degree  the  follicles.  Some  of  the  latter  may  appear 
very  small  and  tubular  and  contain  no  secretion,  while  the  neighboring  fol- 
licles are  enormously  distended  with  masses  of  colloid.  As  usually  seen  in 
sections,  the  colloid  substance  is  homogeneous  or  finely  granular  and  often 
partly  detached   from    the   lining   cells    by  shrinkage.      Vacuoles   are   also 


THE    PARATHYROID    BODIES. 


199 


common.  Although  many  of  these  are  artefacts  and  referable  to  the  action 
of  reagents,  some  preexist  and  contain  materials  of  a  mucous  or  fatty  nature. 
The  blood-vessels  supplying  the  thyroid  tissue  are  unusually  plentiful, 
the  interlobular  arteries,  branches  from  the  superior  and  inferior  thyroids, 
breaking  up  into  close  networks  that  surround  the  follicles  and  lie  imme- 
diately beneath  the  epithelium.  The  lymphatics,  also  abundant,  begin  as 
perifollicular  lymph-spaces,  from  which  are  formed  the  interlobular  lym- 
phatics accompanying  the  blood-vessels.  The  deeper  lymphatics  join  the 
superficial  plexus,  on  the  surface  of  the  organ,  from  which  the  larger  trunks 


I-    l-ollicle 


Interlobular  vessels 


Capillary    network 

surrounding 

lollicle 


Follicle  containing 
colloid 


Fig.  247. — Section  of  injected  body,  showing  rich  capillary  networks  surrounding  follicles.     X  46. 

pass  in  different  directions.  The  nerves  are,  for  the  most  part,  sympathetic 
fibres  supplying  the  walls  of  the  blood-vessels  and  enclosing  the  follicles  in 
plexuses  of  nonmedullated  filaments,  which  end  in  close  relation  with  the 
epithelial  cells. 

THE   PARATHYROID    BODIES. 

These  little  organs,  also  called  the  epithelial  bodies,  when  typically 
present  are  arranged  as  two  pairs,  an  upper  and  a  lower.  The  upper  ones' 
are  the  more  constant  and  usually  lie  against  the  posterior  surface  of  the 
lateral  thyroid  lobes.  The  inferior  bodies  are  less  constant,  both  as  to 
position  and  presence,  sometimes  lying  against  the  side  of  the  trachea  under 
cover  of  the  lower  part  of  the  thyroid  lobes,  or  upon  the  latter,  and  at  other 
times  being  placed  entirely  below  the  thyroid.  The  disposition  of  the  para- 
thyroids may  be  asymmetrical,  in  some  cases  as  many  as  four,  in  others  none, 
lying  on  one  side.  The  bodies  are  6-7  mm.  long,  3-4  mm.  broad,  and  1.5-2 
mm.  thick,  but  may  be  larger  or  smaller.  They  arise  from  the  dorsal  wall 
of  the  third  and  fourth  pharyngeal  furrows  and  thus  difier  from  the  thyroid 
body  in  origin,  as  well  as  structure. 

Each  organ  is  invested  by  a  thin  fibro-elastic  capsule  and  subdivided 
into  uncertain  lobules  by  delicate  sept?,   which  support  the  larger  blood- 


200 


NORMAL   HISTOLOGY. 


Fibrous  capsule 
separating  _ 
thyroid  and 
parathyroid 


Blood-vessel 


Capillar}' 


-Section  including  adjacent  portions  of  human  thyroid 
(above)  and  parathyroid  (below).    X  220. 


vessels.  The  distinctive  tissue  consists  of  closely  placed  polygonal  epithelial 
cells  (10  p.  in  diameter),  disposed  as  continuous  masses  or  as  imperfectly 
separated  cords  and  alveoli.  The  cells  have  round  nuclei  and  are  lodged 
within  a  reticulum  composed  of  wide  capillaries  and  delicate  strands  of  fibro- 

elastic  tissue,  the  whole 
a  J  y/  \^  often  bearing  a  striking 
'^\-^x  It'  likeness  to  the  anterior  lobe 
of  the  pituitary  body  (page 
309),  even  to  the  presence 
of  colloid  substance  within 
some  of  the  alveoli.  When 
the  cell-masses  tend  tow- 
ards the  alveolar  type,  the 
epithelium  and  the  blood- 
channels  are  in  intimate 
relation,  an  arrangement 
probably  facilitating  the 
distribution  of  the  partic- 
ular product  of  the  cells. 
The  significance  of  the 
parathyroid  bodies  as  dis- 
tinct organs,  and  not  as 
merely  masses  of  modified 
thyroid  tissue,  has  been 
established  by  both  an- 
atomical and  physiologi- 
cal investigations ;  like  the 
thyroid,  they  are  ' '  ductless  glands ' '  and  organs  of  internal  secretion. 
The  blood-vessels  supplying  the  organs  are  the  minute  parathyroid 
arteries,  usually  from  the  branches  of  the  inferior  thyroid,  to  each  one  of 
which  a  body  is  attached.  The  capillaries  are  relatively  wide  and  ramify 
between  the  nests  of  cells.  Little  is  known  concerning  the  lymphatics  and 
nerves;  the  latter,  however,  are  chiefly  sympathetic  fibres  for  the  walls  of 
the  blood-vessels. 

THE   THYMUS    BODY. 

Although  actually  increasing  in  size  and  weight  until  towards  puberty, 
the  thymus  body  is  essentially  an  organ  of  very  early  childhood,  attaining 
its  highest  development  about  the  second  year.  At  that  time  it  stretches 
from  the  root  of  the  neck  downwards  into  the  thorax,  behind  the  sternum  and 
over  and  in  front  of  the  pericardium,  to  about  the  line  of  the  fourth  costal 
cartilage.  It  is  thickest  above  and  descends  as  two  flattened  irregular  lobes, 
separated  by  fibrous  tissue,  of  which  the  left  one  is  more  often  the  larger. 
Subsequently  more  or  less  extensive  atrophy  and  replacement  of  the  thymus- 
tissue  occurs,  variable  islands  of  the  latter  surrounded  and  invaded  by  fat- 
cells  being  the  usual  condition  of  adolescence.  Notwithstanding  this  replace- 
ment by  adipose  and  connective  tissue,  the  thymus  never  entirely  disappears, 
remains  of  its  tissue  being  present  even  in  extreme  old  age. 

The  thymus  body  develops  from  paired  epithelial  outgrowths  from  the 
ventral  wall  of  the  third  pharyngeal  furrows.  From  these  result  long  cylin- 
drical masses  of  closely  packed  epithelial  cells,  which  grow  downwards  and 
for  a  time  enclose  a  lumen  that  later  disappears.  The  masses  increase  by 
solid  outgrowths,  resembling  those  of  an  immature  tubo- alveolar  gland,  so 


THE  THYMUS    BODY. 


20I 


that  by  the  middle  of  foetal  life  the  organ  has  acquired  a  lobulated  structure, 
a  condition  intensified  by  the  ingrowth  of  vascular  mesodermic  tissue. 
Meanwhile,  the  original  closely  packed  epithelial  elements  undergo  marked 
change,  most  being  converted  into  stellate  cells  that  form  a  reticulum. 
From  other  cells  arise  by  repeated  division  a  profusion  of  very  small  cells 
that  fill  the  meshes  of  the  reticulum  produced  by  the  transformation  just 
mentioned.  The  genetic  relation  of  the  small  cells  to  the  original  ento- 
dermic  epithelium  is  still  disputed.  According  to  Stohr,  Bell  and  others, 
they  arise  from  the  epithelial  elements;  according  to  Hammar  and  others, 
they  are  mesodermic  cells  that  early  enter  the  thymus  and  correspond  to 


.fd:     "liik^.m^^' 


Fig.  249.— Section  of  fleveloping  thymus  from  human  foetus  of  third  month  ;  among  the  stellate  reticulum- 
cells  are  seen  the  small  thymic  lymphocytes.      <  690.     (Hammar.) 


true  lymphocytes.  All  are  agreed,  however,  that  the  "small  cells"  — 
the  thymic  lymphc^cytes — closely  resemble  morphologically  the  ordinarv 
lymphocytes. 

The  thymus  is  invested  by  a  loose  fibro-elastic  capsule,  from  which 
septa,  rich  in  blood-vessels  and  lymphatics,  pass  inwards  and  subdivide  the 
organ  into  a  number  of  indefinite  lobes.  The  latter  are  broken  up  by  partial 
partitions  into  lobules,  in  which  a  denser  peripheral  tract,  the  cortex,  and  a 
lighter  central  one,  the  viediilla,  can  be  distinguished,  although  these  divi- 
sions are  often  not  sharply  defined.  The  thymus  possesses  no  duct-system 
and,  hence,  is  often  classed  as  a  "  ductless  gland. ' ' 

The  cortex  consists  of  closely  packed  small  cells  (7-10  <,'■  in  diameter), 
whose  cytoplasm  is  so  meagre  that  the  deeply  staining  nuclei  are  their  most 
evident  parts.  The  small  cells  are  supported  by  a  delicate  meshwork  formed 
by  the  stellate  reticithim-cells.  Numerous  capillary  blood-vessels,  with 
accompanying  scanty  strands  of  fibrous  tissue,  are  intermingled  with  the 
cortical  elements.  The  medulla  is  of  looser  texture  and  contains,  in  addi- 
tion to  the  small  and  reticulum-cells,  much  larger  epithelial  cells,  either 
smgly  or  in  limited  groups  and  cords. 

The  most  distinctiv^e  feature  of  the  medulla — and,  indeed,  of  the  entire 
organ,  which  otherwise  bears  a  general  resemblance  to  lymphoid  tissue — is 
the  presence  of  the  irregularly  spherical  or  elongated  thymic  bodies,  or 
corpuscles  of  Hassall,  which  appear  as  small  lighter  areas  scattered  through- 
out the   medulla.      They   first  appear  about  the   middle   of  foetal   life  and 


202 


NORMAL   HISTOLOGY. 


increase  in  number  and  size  until,  at  the  end  of  the  first  year,  they  are  plentiful 
and  may  attain  a  diameter  of  from  .  2-.  3  mm. ,  although  usually  they  measure 


.»»/•  *:..  ''^ 


Lobule 


Thymic, 
bodj 


Cortical  follicles 


''  '*'  -        Interlobular  septum 

I  { I  ^'^        Blood-vessels, 


--^^4o' j:r    ^//^r'^ 


^^-i-".  r. ,  "'  / 


Y^ 


,5 


Capsule 

Fig.   250.— Transverse   section   of    lobe   of   thymus   body   of    child,   showing   general   arrangement  of 

lobules.    X  20. 

much  less  (40-60  <)).     These  bodies,  present  only  in  the  medulla,  are  com- 
posed of  large  concentrically  disposed  epithelial  elements,  which,  flattened 


■Capsule 


^-^^■x Cortex 


within  medulla 


Fig.  251. — Section  of  thymus,  showing  details  of  cortical  and  medullary  substance.    X  200. 

and  connected  with  the  reticulum-cells  at  the  periphery  of  the  corpuscle,  at 
its  centre  exhibit  evidences  of  degeneration,  such  as  nuclei  poor  in  chromatin 
or  disappearing,    breaking  down  of  cytoplasm  or  invasion  of  the  cells  by 


THE   THYMUS    BODY. 


20^ 


leucocytes.  The  centres  of  the  bodies  may  enclose  vacuoles  containing  fat, 
or  mucus-reacting  or  colloid  substance — the  latter  by  some  being  regarded 
as  representing  the  special  production  of  the  organ. 

The  significance  and  ijenetic  relations  of  the  thymic  bodies  have  long  been  the 
subject  of  discussion.  Formerly  they  were  regarded  as  the  remains  of  the  original 
epitliehal  elements  of  the  organ,  all  other  parts  being  the  products  of  the  invading 
mesoderm.  In  the  light  of  present  embryological  data,  it  is  probable  that  the  cor- 
l>u.scles  result  from  the  aggregation  of  hypertrophied  and  otherwise  modified  reticulum- 
cells  and,  therefore,  that  they  are  new  and  special  formations,  distinctive  of  the  organ, 
and  not  merely  the  atrophic  remains  of  the  original  epithelial  elements.  It  is  evident, 
however,  that  they  are  indirectly,  through  the  reticulum-cells,  derivatives  of  the  primary 
epithelium.  .Since  new  corpuscles  are  being  continually  formed  and  those  existing 
increase  in  size,  during  the  active  period  of  the  thymus,  it  is  possible  that  they  are 
concerned  in  producing  a  substance  ser\ing  some  particular  purpose  during  early  life. 


.Thymus 
^         y^     tissue 


Fig.  252.- 


-Section  of  thymus  body  of  man  of  twenty-eiglit,  showing  nivasion  and  replacement  of  thymus- 
tissue  by  fat.      <  20. 


The  assumed  importance  of  the  thymus  as  the  producer  of  the  first  lymphocytes, 
credited  to  the  organ  by  some,  is  doubtful.  There  is  no  convincing  evidence  that  the 
thymus  is  the  seat  of  red  blood-cell  formation. 

In  addition  to  numerous  capillaries,  leucocytes  and  eosinophiles  are  present 
among  the  constituents  of  the  medulla.  As  age  advances,  the  small  cells  become 
less  numerous  and  the  corte.x  markedly  diminishes  in  thickness,  so  that  the  medullary 
substance  comes  into  relation  with  the  surrounding  vascular  interlobular  connective 
tissue  with  increasing  frequency  and  e.xtent.  At  a  variable  time,  in  some  cases  before 
the  second  year  and  in  others  not  until  much  later,  an  active  general  regression  and 
atrophy  of  the  thymus  becomes  established.  The  general  process,  however,  is  often 
antedated  by  reduction  in  the  thymus,  probably  associated  with  impaired  nutrition, 
whereby  the  number  of  the  small  lymphoid  cells  is  greatly  decreased  and  the  distinction 
between  cortex  and  medulla  disappears.  The  medulla  is  the  seat  of  occasional  small 
cysts,  of  uncertain  form  and  size,  lined  with  epithelial  elements  that  often  bear  groups 
of  cilia-like  processes. 

Coincident  with  the  atrophy  of  the  thymus-tissue,  fat-cells  progressively 
appear  in  the  interlobular  tracts,  Avhich  latter,  in  consequence,  become  in- 


204  NORMAL   HISTOLOGY. 

creasingly  more  voluminous,  with  separation  of  and  encroachment  upon  the 
diminishing  thymic  tissue.  Gradually  the  latter  is  replaced  by  the  adipose 
tissue,  until  only  isolated  islands  of  the  characteristic  thymus-tissue  remain 
(Fig.  252).  Complete  disappearance  of  this  structure,  however,  is  very 
exceptional,  even  in  advanced  age  a  certain  amount  of  it  being  recognizable 
upon  microscopical  examination. 

The  blood-vessels  distributed  to  the  thymus  send  their  twigs  into  the 
organ  in  such  manner,  that  these  lie  at  the  junction  of  the  cortex  and  medulla 
(Fig.  250).  Capillaries  thence  proceed  to  the  cortex  and  medulla,  those 
within  the  former  being  more  abundant  than  those  distributed  to  the  medulla. 
The  venous  radicles  begin  in  the  thymus-stroma,  some  passing  through  the 
medulla  while  others  become  more  directly  tributaries  of  the  interlobular 
veins.  The  lymphatics  are  numerous  and  represented  by  networks  of 
lymph-spaces  close  to  the  periphery  of  the  lobules,  from  which  are  formed 
the  more  definite  interlobular  lymphatic  vessels,  that  in  turn  drain  into  the 
large  efferent  trunks.  The  existence  of  intralobular  passages,  corresponding 
to  lymph-sinuses,  has  not  been  established.  The  nerves  are  small  and  are 
derived  from  the  sympathetics  and  the  vagi.  The  fibres  are  traceable  along 
the  interlobular  septa,  in  company  with  the  blood-vessels,  to  whose  walls 
they  are  chiefly  distributed.  A  very  meagre  number  of  nonmedullated  fibres 
have  been  described  as  terminating  within  the  medulla  as  free  endings. 


THE   URINARY   ORGANS. 

These  organs  include  the  kidneys,  the  glands  which  secrete  the  urine, 
the  ureters,  the  canals  which  collect  the  urine  and  con\-ey  it  from  the  kidneys 
to  the  bladder,  the  receptacle  in  which  the  urine  is  temporarily  stored,  and 
the  urethra,  the  passage  through  which  the  urine  is  discharged. 

THE    KIDNEYS. 

The  kidneys  are  two  flattened  ovoid  glands,  of  peculiar  bean-shaped 
form,  deeply  placed  within  the  abdominal  cavity  against  its  posterior  walls, 
one  on  each  side  of  the  lumbar  spine.  They  are  invested  by  a  thin  fibrous 
capsule,  which  is  distinct  from  the  renal  tissue  that  it  covers  and  is  exposed  only 
after  its  removal.  The  mesial  border  of  each  kidney  is  interrupted  by  a  slit- 
like opening,  the  hiluin,  which  leads  into  a  more  extended  but  flattened 
space,  the  sinus,  enclosed  by  the  surrounding  substance  of  the  kidney.  In 
addition  to  the  blood-vessels,  lymphatics,  and  nerves  passing  to  and  from 
the  kidney  through  the  hilum,  the  sinus  contains  the  upper  expanded  end 
of  the  ureter,  which  also  emerges  at  the  hilum.  The  interspaces  between 
these  structures  are  filled  with  loose  fatty  areolar  tissue. 

Architecture  of  the  Kidney. — Before  describing  its  histological 
details,  it  will  be  of  advantage  to  consider  the  general  plan  upon  which  the 
kidney  is  built — its  architecture  as  contrasted  with  its  structure.  The  entire 
organ — a  conspicuous  example  of  a  compound  tubular  gland — is  made  up  of 
a  number  of  divisions  which  in  the  mature  condition  are 
so  closely  blended  as  to  give  little  evidence  of  the  striking 
subdivision  or  lobulation  of  the  foetal  kidney.  The  exter- 
nal surface  of  the  latter  (Tig.  253)  is  broken  up  by 
furrows  into  a  number  of  polygonal  areas,  each  of  which 
represents  the  base  of  a  pyramidal  mass  of  renal  sub- 
stance, the  kidney  lobe,  separated  from  its  neighbors 
by  connective  tissue.  It  includes  the  entire  thickness 
of  the  organ,  between  the  exterior  and  the  sinus,  and 
ends  internally  in  a  conical  apical  projection,  the  renal 
papilla.  Shortly  after  birth,  the  lobulation  gradually  ,ew-born''cTuci^"s7ow- 
disappears  on  the  surface,  which  becomes  smooth,  the  ing  areas  on  surface 
interlobar  connective  tissue  septa  within  the  organ  likewise  marriobes!"^  °  ^'^'' 
disappearing,  while  the  papillae  alone  remain  as  indications 
of  the  original  subdivisions.  Although  the  outlines  of  the  lobes  occasionally 
persist  on  the  surface  of  the  adult  human  kidney,  in  many  of  the  lower 
animals  (reptiles,  birds,  ruminants,  cetaceans  and  certain  carnivora)  the 
subdivisions  are  normally  retained.  In  some  mammals  (rodents  and  insec- 
tivora)  the  entire  kidney  corresponds  to  a  single  papilla,  while  in  others 
(elephant  and  horse)  no  distinct  papillae  exist. 

On  examining  the  cut  surface  of  the  kidney,  opened  by  a  longitudinal 
section  passing  from  the  convex  border  through  the  sinus  (Fig.  254),  the 
papillae  are  seen  to  form  the  free  apices  of  conical  areas,  the  renal  pyramids, 
whose  bases  lie  embedded  within  the  surrounding  kidney-substance  compos- 
ing the  outer  third  of  the  organ.  This  peripheral  zone,  which  in  the  fresh 
kidney  appears  darker  and  granular  when  compared  with  the  lighter  and 

205 


2o6 


NORMAL    HISTOLOGY 


Cortex 


Renal  papilla 


Striated  renal  pyramid,  is  the  cortex.  The  medulla  includes  the  conical 
areas  of  the  pyramids  and  partially  occupies  the  inner  two  thirds  of  the 
thickness  of  the  organ.  The  cortex  constitutes  the  bulk  of  the  kidney, 
forming  the  entire  surface,  including  the  lips  of  the  hilum,  and  receiving 
and  surrounding  the  bases  of  the  pyramids.  The  cortical  tissue,  further, 
penetrates  between  the  pyramids,  separating  them  and  in  places  gaining  the 
sinus.  These  interpyramidal  extensions  are  the  renal  columns,  or  columns 
of  Bertin,  and  consist  of  typical  cortical  substance.  Since  the  branches  of  the 
renal  blood-vessels  lie  within  the  interlobar  connective  tissue  separating 
the  primary  subdivisions  of  the  foetal  organ,  these  vessels  never  enter  the 
kidney-substance  by  passing  into  the  papillae,  but  always  at  the  side  of  and 

between  these.  They  sink 
into  the  renal  substance, 
therefore,  through  the  areas 
occupied  by  the  renal  col- 
umns, the  free  surfaces  of 
which  are  pitted  by  the 
vascular  foramina. 

On  inspection  with  a 
hand-glass,  it  will  be  seen 
that  the  cortex  is  not  uni- 
form, but  subdivided  into 
radially  disposed  darker 
and  lighter  tracts.  The  lat- 
ter, longitudinally  striated 
and  wedge-shaped,  are  the 
medullary  rays,  or  pars 
radiata,  since  they  are  ap- 
parently continuations  of 
the  medullary  tissue.  The 
darker  tracts,  between  the 
medullary  rays,  constitute 
the  labyrinth,  or  pars  con- 
voluta,  and  appear  granular 
owing  to  the  tortuous  course 
of  the  component  urinife- 
rous  tubules.  The  labyrinth 
is  studded  with  bright  red  points  marking  the  position  of  minute  vascular 
tufts  or  glojneruli;  these  are  limited  to  the  labyrinth  and,  therefore,  never 
present  within  the  medullary  rays  or  the  renal  pyramids,  although  found 
within  the  columns  of  Bertin. 

On  sectioning  minutely  injected  organs,  it  will  be  observed  that  the 
larger  interlobar  arteries,  on  gaining  the  boundary  zone  between  the 
cortex  and  the  medulla,  break  up  into  smaller  branches,  some  of  which  pass 
towards  the  surface,  while  others  change  their  direction  and  assume  a  more 
horizontal  course.  The  terminal  twigs — "end-arteries,"  since  anastomoses 
are  wanting — run  generally  perpendicular  to  the  exterior  of  the  kidney  and 
occupy  the  centres  of  the  tracts  separating  the  medullary  rays.  The  latter 
are,  therefore,  the  axes  of  minute  conical  masses  of  renal  substance,  the 
cortical  lobules,  whose  bases  He  at  the  surface  of  the  kidney  and  the  apices 
within  the  pyramids  of  the  medulla.  From  the  foregoing  it  is  evident  that 
each  renal  pyramid  receives  a  group  of  cortical  lobules,  the  component 
tubules    of   which,    on    entering    the    pyramid,    become   progressively   less 


Part  of  sinus 

Renal  papilla, 
apex  of 
pyramid 


Ureter 


Fig.  254. — Longitudinal  section  of  kidney,  showing  divisions  of 
renal  substance  and  relations  of  pelvis  and  calyces. 


THE    KIDNEY. 


207 


numerous  but  larger,  in  consequence  of  repeated  union,  until,  as  the  ^^ide 
papillary  ducts,  they  end  at  the  summit  of  the  renal  papilla. 

The  Kidney-Substance. — The  fundamental  components  of  the  ver- 
tebrate excretory  organ,  both  in  the  foetal  and  mature  condition,   include: 

( 1 )  a  tuft  of  arterial  capillaries  derived  more  or  less  directly  from  the  aorta ; 

(2)  tubules  lined  with  secretory  epithelium ;  and   (3)  a  duct  for  conveving 
the    excretory    products. 

These        constituents        are  Lab>Timh  Med.  ray  Labyna.h 

represented  in  the  kidney 
of  man  and  the  higher  an- 
imals by:  (i)  the  gloincr- 
2cliis,  (2)  the  convoluted 
iirinifero7is   tubules,    and 

(3)  the  collecting  tubules 
and  the  ureter  composing 
the  duct-system.  Since, 
in  a  general  way,  to  the 
epithelium  may  be  as- 
cribed the  function  of  tak- 
ing from  the  circulation 
the  more  solid  constitu- 
ents of  the  urine,  and  to 
the  glomerulus  the  secre- 
tion of  the  watery  parts, 
obviously  the  most  favor- 
able arrangement  to  se- 
cure the  removal  of  the 
excretory  products  is  one 
insuring  flushing  of  the 
entire  tubule  with  the  fluid 
secreted  by  the  glomeru- 
lus. Such  an  arrangement 
implies  the  location  of  the 
\ascular  tuft  at  the  \ery 
beginning  of  the  tubule — 
a  disposition  which  in  fact 
is  found  in  the  kidneys  of 
all  higher  animals.  The 
number  of  the  glomeru- 
li, therefore,  corresponds 
with  that  of  the  urinifer- 
ous  tubules,  each  of  which 
begins  in  close  relation 
with  a  vascular  tuft.  The  kidney-substance  consists  of  an  intricate,  although 
definitely  arranged,  complex  of  uriniferous  tubules,  supported  by  an  inter- 
stitial connective  tissue  stroma,  which  have  their  commencement  in  the  cortex 
and  their  termination  at  the  apices  of  the  papillae,  their  intervening  course 
being  marked  by  many  and  conspicuous  variations  in  the  character,  size,  and 
direction  of  the  tubules. 

The  uriniferous  tubule  begins  as  a  greatly  expanded  blind  extremitv, 
the  capsule  (i),  which  surrounds  the  vascular  tuft  or  glomerulus,  the  two 
together  constituting  the  Malpighiayi  body  or  renal  corpuscle,  which  lies 
within  the  labyrinth.      On  leaving  the  Malpighian  body,  the  tubule  becomes 


Papillary  duct  ^. 


Loop  of 
Henle 


Papilla 
Fig.  255. — Diagram  illustraling  the  course  of  a  uriniferous  tubule. 


.^ 


208 


NORMAL    HISTOLOGY. 


Capsule 


Malpighian 
body  in 
labyrinth 


\ery  tortuous  and  arches  towards  the  free  surface  as  k\\q.  pi'oximal  convohited 
tubule  (2);  this,  after  a  course  of  considerable  length,  usually  leaves  the 
labyrinth  and  enters  the  medullary  ray,  which  it  traverses,  somewhat  reduced 
in  diameter  and  slightly  winding  in  course,  and  passes  into  the  medulla. 
Immediately  upon  gaining  the  latter,  the  tubule  becomes  markedly  nar- 
rowed, penetrates  the  re- 
nal pyramid  for  a  variable 
distance  towards  the  pa- 
pilla, then  bends  sharply 
upon  itself  and  retraces  its 
course  to  enter  once  more 
the  labyrinth.  Its  excur- 
sion into  the  medulla  in- 
cludes the  descending  limb 

(3)  and    ascendi7ig   limb 

(4)  of  the  loop  of  Henle. 
The  ascending  limb — the 
longer  and  v.ider  of  the 
parallel  limbs  of  the  loop 
—  rises  within  the  labyrinth 
to  the  immediate  vicinity 
of  the  corresponding  Mal- 
pighian body  and  then, 
after  arching  over  or 
around  the  body,  gives 
place  to  the  distal  convo- 
hited tubule  (5),  a  segment 
which,  marked  by  in- 
creased diameter  and  tor- 
tuosity, crosses  the  general 
path  of  the  proximal  con- 
voluted tubule  and  is  suc- 
ceeded by  the  narrower 
arching  connecting  tubule 
(6).  The  latter  soon  en- 
ters the  medullary  ray  and 
joining  with  similar  canals, 
takes  part  in  forming  the 
straight  collecting  hibule 
(7),  which,  progressively 
increasing  in  size  by  junc- 
tion with  others,  traverses 
the  remaining  length  of 
the  medullary  ray  and  en- 
ters the  pyramid.  Within  the  deeper  part  of  the  latter,  the  collecting 
tubules  fuse  into  larger  and  larger  canals  until,  as  the  relatively  \\'\d&  papillary 
ducts  (8),  they  terminate  at  the  apex  of  the  papilla  at  the  orifices,  \h& papil- 
lary fora7ni7ia,  which  open  into  the  calyces,  as  the  subdivisions  of  the 
expanded  beginning  of  the  renal  duct  are  called. 

Although  as  a  matter  of  convenience  the  entire  canal,  from  its  com- 
mencement at  the  Malpighian  body  to  its  termination  on  the  papilla,  has 
been  described  as  the  uriniferous  tubule,  both  genetically  and  functionally  two 
distinct  parts  should  be  recognized.      These  are  (a)  the  uriniferotis  tubule 


-Section    of   cortex,  showinjr   relation    of    labyrinth   to 
medullary  rays.     X  50. 


THE    KIDNEY. 


20Q 


proper,  which  includes  all  the  conventional  subdivisions  from  the  Malpighian 
body  to  the  termination  of  the  distal  convoluted  tubule;  and  ( b)  the  diict-tiidc, 
which,  when  traced  from  the  ])apilla  towards  the  cortex,  undergoes  repeated 
division  imtil,  from   the  single  main 


Capsule 


Stem,  the  number  of  connecting  tu- 
bules is  sufificient  to  provide  each 
uriniferous  tubule  proper  with  an 
excretory  canal. 

Details  of  the  Uriniferous  Tubule. 
— The  (general  course  and  relations  of 
the  tubule  having  been  sketched,  a  brief 
account  of  its  more  important  structural 
details  may  here  fi  nd  an  appropriate  place. 

I.  The  Malpighian  body,  irregularly- 
spherical  and  from  .12-. 20  nmi.  in  diam- 
eter, consists  of  the  glomerulus  and  the 
capsule.  The  gloiiierulns  is  an  aggrega- 
tion of  tortuous  capillary  blood-vessels 
derived  from  the  lateral  terminal  branches 
given  off  from  the  cortical  arteries  as  these 
pass  between  the  lobules  towards  the  free 
surface  of  the  kidney.  One  of  the  lat- 
eral branches, very  short  and  often  arched, 

enters  the  adjacent  Malpighian  body  as  the  vas  affercns,  where  it  divides  into  from  foiir 
to  si.\  twigs,  each  of  which  breaks  up. into  capillaries.  These  may  anastomose  and  form 
a  vascular  comple.\,  or  each  terminal  twig  may  give  rise  to  an  isolated  capillary  territory, 


Fig.  257. — Injected  glomerulus,  showing  afferent 
and  efferent  vessels  and  continuation  of  the  latter 
into  the  intertubular  capillary  network.      ■'_  i8o. 


Con\oluted  tubule 


Capsule 


Convoluted  t 


ubules^  ^..V-^^M*       ^y^  ^ 


Fig.  25S. — Section  of  cortex,  showing  details  of  a  renal  corpuscle  or  Malpighian  body;  the  glomerulus  is 
surrounded  by  capsule  which  passes  into  obliquely  cut  neck  of  tubule.     X  200. 


the  entire  glomerulus  then  consisting  of  a  group  of  vascular  lobules.  The  channels  of 
e.xit  unite  to  form  the  single  vas  efferens,  through  which  the  blood  from  the  glomerulus 
escapes.  As  the  efferent  emerges  from  the  body,  it  is  close  to  the  afferent  vessel,  both 
usually  lying  on  the  side  opposite  to  that  from  which  the  tubule  springs.  The  capsule, 
the  dilated  beginning  of  the  uriniferous  tubule,  almost  completely  invests  the  glomerulus 
with  a  double  layer,  the  prolonged  wall  of  the  tubule.  The  outer  and  inner  layers  are 
14 


2IO 


NORMAL    HISTOLOGY. 


continuous  around  the  narrow  stalk,  through  which  the  vessels  pass  and  at  which  the 
reflection  of  the  capsule  is  incomplete.  The  inner  ("  visceral  ")  la3'er  is  firmly  attached 
to  the  glomerulus  by  delicate  strands  of  connective  tissue  which  likewise  hold  together 
the  capillaries.     The  capsule  consists  of  a  membrana  propria  lined  with  a  single  layer 


Blood-vessel 


Fig.  259. — Convoluted  tubules,  cut 
transversely  and  obliquely,  showing 
character  of  epithelial  lining.    X  280. 


Fig.  260. — Portion  of  medullary  ray,  showing  "spiral"  part 
of  convoluted  and  collecting  tubules.    X  280. 


of  flat  polyhedral  epithelial  cells,  directly  continuous  with  the  epithelium  of  the  tubule. 

2.   The  proximal  convoluted  tubule  begins  at  the  constriction,  the  neck,  of  the 

capsule  and   abruptly  widens  into  the  tortuous  segment  that  forms  approximately 

one   fifth  of  the  entire    length  of    the  tubule.      Its  diameter  varies   from  40-60  fi. 


Ascending  f^    /g 

limb'  [(  ^ '/ 

Blood-vessel 


Blood-vessel 


Loop. 


Fig.  261. — Longitudinal  section  of 
medulla,  showing  parts  of  limbs  of 
Henle's  loop.    X  280. 


Collecting  tubule 


Fig.  262. — Longitudinal   section  of  medulla 
passing  through  Henle's  loop.    X  280. 


In  common  with  other  parts  of  the  renal  tubule,  its  wall  consists  of  a  homogeneous 
basement  membrane  lined  with  a  single  layer  of  epithelial  cells.  The  latter,  in  the 
convoluted  tubule,  are  not  defined  by  sharp  outlines,  but  more  or  less  blended  into 
a  continuous  protoplasmic  sheet,  a  form  of  syncytium,  in  which  the  spherical  nuclei, 
which  lie  near  the  basement  membrane,  indicate  the  approximate  extent  of  the 
individual  low  columnar  cells.  Although  subject  to  much  and  inconstant  variation, 
their  cytoplasm   exhibits  a  differentiation  into  a  broad  darker  outer  and  a  narrow 


THE   KIDNEY. 


211 


lighter  t?mer  zone.  The  former  is  marked  by  coarse  radial  striations,  the  so-called 
rods,  produced  by  parallel  rows  of  granules  along  the  protoplasmic  threads.  The 
narrow  inner  zone,  next  the  lumen,  is  relatively  clear,  containing  few  granules  and 
showing  a  faint  striation,  due  probably  to  the  arrangement  of  the  protoplasmic  threads. 
This  "bristle  border,"  as  it  is  sometimes  called,  is  seen  only  in  very  well  preserved 
tissue;  since  it  is  prone  to  disintegrate,  the  partially  destroyed  border  may  give  rise  to 
appearances  mistaken  for  cilia.  During  active  secretion,  the  epithelial  cells  are 
relatively  low  and  the  lumen 
of  the  tubule  is  wide,  these 
relations  being  reversed  dur- 
ing periods  of  functional  inac- 
tivity. 

3.  The  loop  of  Henle  be- 
gins in  the  boundary  zone 
between  the  cortex  and  me- 
dulla by  the  passage  of  the 
end-segment  of  the  convoluted 
tubule  into  the  desce7tding  limb, 
which  is  distinguished  not  only 
by  the  conspicuous  reduction 
in  its  diameter  (12-15  //),  being 
the  narrowest  part  of  the  entire 
renal  tubule,  but  also  by  the 
character  of  its  epithelium. 
The  latter  consists  of  low  flat- 
tened elements,  in  which  the 
ellipsoidal  nuclei  equal  or  sur- 
pass the  thickness  of  the  cells, 
whose  cytoplasm  is  clear  or 
slightly  granular.  The  ascend- 
ing liinb  differs  from  the  de- 
scending in  its  increased  diam- 
eter (24-28 /u),  thicker  epitheli- 
um, which  is  dark  and  striated, 
and  extension  into  the  cortex. 
Since  the  cuboidal  cells  are 
often  irregular  in  height,  the 
lumen  correspondingly  varies, 
in  places  being  almost  oblite- 
rated. The  length  of  Henle' s 
loop  is  influenced  by  the  level 
of  the  corresponding  Malpighi- 
an  body  within  the  cortex — the 
nearer  the  medulla  the  body 
lies  the  greater  the  descent  of 
the  loop  towards  the  papilla, 
and  vice  versa.  On  entering 
the  cortex  the  ascending  limb  rises  to  the  immediate  vicinity  of  its  IMalpighian  body, 
around  or  over  which  it  curves  to  end  in  the  succeeding  distal  convoluted  tubule.  The 
usual  position  of  the  sudden  transition  from  the  narrow  into  the  wider  part  of  Henle' s 
loop  is  in  the  descending  limb  a  short  distance  above  the  loop,  although  the  change 
may  occur  beyond  the  turn,  or  even  within  the  bend  itself. 

4.  The  distal  convoluted  or  intermediate  tubule,  from  40-45  fi  in  diameter,  pur- 
sues a  moderately  tortuous  path,  marked  by  a  number  of  abrupt  changes  in  direction, 
but  in  a  general  way  is  enclosed  by  the  arch  of  the  proximal  convoluted  segment 
which  it  finally  crosses.  Its  epithelium,  which  at  first  resembles  that  of  the  ascending 
limb,  becomes  clearer  and  less  distinctly  striated,  the  cells  having  a  more  definitely 
defined  low  cylindrical  or  pyramidal  form,  although  presenting  local  variations  in 
height. 


Fig.  263.  —  Longitudinal  section  of  renal  pyramid,  showing 
general  structure  of  medulla  with  Henle's  loops  and  collecting 
tubules.    X  45. 


212 


NORMAL    HISTOLOGY. 


5.  The  connecting  or  junctional  tubule  effects  the  union  of  the  uriniferous  tubule 
proper  with  the  duct-system.  It  is  narrower  (23-25  fj)  than  the  preceding  segment  and 
lined  with  well  defined  cuboidal  cells,  which  being  lower  afford  an  increased  lumen. 

After  an  usually  short  and  somewhat 
arched  course,  the  connecting  ttibule 
enters  the  medullar}'  ray  and,  uniting 
with  similar  canals,  joins  in  forming 
the  collecting  tubule. 

6.  The  collecting  tubule  at  first 
lies  within  the  medullary  raj%  where 
it  represents  the  beginning  of  the 
system  of  straight  duct-tubes  opening 
on  the  papilla,  and  then  passes  into  the 
renal  pyramid.  During  their  course 
through  the  medullary  ray,  the  collect- 
ing tubules  repeatedly  unite  to  produce 
stems,  which,  while  increasing  four-  or 
five-fold  in  diameter,  diminish  in  num- 
ber as  they  descend  in  the  medulla. 
In  con.sequence  of  this  fusion,  within  the 
pyramid  the  collecting  tubules  are  dis- 
posed in  groups,  each  of  which  corre- 
sponds to  the  tubules  prolonged  from  a  single  medullary  ray,  surrounded  by'  the 
limbs  of  the  loops  of  Henle.  The  groups  are  further  separated  by  the  bundles  of  straight 
blood-vessels    [vasa  recta-)   of   the   medulla.     The   epithelium  lining  the   collecting 


Fig.  264. — Section  across  upper  part  of  renal  pyra- 
mid, showing  groups  of  blood-vessels  surrounded  by 
uriniferous  tubules.     X  50. 


Blood-vessel 


;«o«y '*iJ>-^  /J  -xV\^>^/r^>k'.  'I  '  ^c.Q^r  Descending 
f^jooy  ^-/]\o  °\  r;;  \^ o  o  o^,;ii«--^^^l^^  hmb  of  loop 


A  J 


.t"^^/ 


/  »    , 


4- Ascending 
'^;.VL      limb  of  loop 


L. 


■  Collecting 
tubule 


o 


f  ^^'h 


00 '^'^y 


Fig.  265. — Section  of  medulla  across  renal  pyramid,  showing  large  collecting  tubules,  limbs  of  Henie's 
loops,  blood-vessels  and  interlubular  stroma.     ,<  130. 

tubules  consists  of  clear,  distinctly  defined,  cuboidal  cells,  whose  height  gradually 
increases  as  the  medulla  is  traversed.  After  converging  to  within  about  5  mm.  of  the 
apex  of  the  papilla,  the  now  large  collecting  tubules  undergo  repeated  junction, 
increasing  in  diameter  (50-60  //)  but  rapidly  diminishing  in  number,  to  form  the  wide 
papillary  ducts. 


THE    KIDNEY.  213 

7.  The  papillary  ducts,  tlie  final  segment  of  the  renal  tubule,  number  from  ten  to 
eighteen  for  each  papilla,  at  the  apex  of  whicli  they  open  into  the  calyx.  Each  is 
formed  by  the  junction  of  from  ten  to  thirty  of  the  larger  collecting  tubules  and  attains 
a  diameter  from  200-300  fi.  The  lining  epithelium  is  composed  of  conspicuous  clear 
columnar  cells,  about  20  «  in  height  and  one-third  as  much  in  width,  wiiich  rest  upon 
a  basement  meinbrane  almost  as  far  as  the  end  of  tiie  canal.  At  this  point  the  simple 
columnar  epithelium  of  the  duct  becomes  continuous  with  the  stratified  squamous 
epithelium  that  covers  tlie  free  surface  of  the  papilla  and  lines  tlie  calyx. 

The  Supporting  Tissue. — The  uriniferous  tubules  and  the  blood-vessels  are  held 
in  place  by  a  delicate  interstitial  stroma  of  reticular  connective  tissue,  elastic  fibres 
being  relatively  very  few.  At  the  surface  of  the  kidney  this  tissue  is  condensed  into 
a  compact  fibrous  stratum,  the  iiutica  albuginca,  containing  scattered  bundles  of 
unstriped  muscle  and  an  increasing  number  of  elastic  fibres,  not  to  be  confounded 
with  the  fibrous  tunic  that  envelops  the  organ  and  may  be  stripped  off  without  dis- 
turbing the  renal  substance.  Although  forming  a  continuous  framework  throughout 
the  kidney,  the  interstitial  stroma  is  not  uniformly  distributed,  being  most  abundant 
along  the  path  of  the  interlobar  and  the  larger  blood-vessels,  from  whose  adventitia 
delicate  trabecuke  extend  in  all  directions  to  form  the  meshes  lodging  the  tubules,  the 
smaller  vessels  and  the  capillaries.  Within  the  cortex  the  supporting  tissue  is  meagre, 
being  best  developed  along  the  interlobular  vessels  and  around  the  Malpighian 
bodies.  The  interstitial  tissue  is  much  more  plentiful  within  the  medulla  than  the 
cortex,  its  amount  increasing  towards  the  apex  of  the  papilla,  where  considerable 
tracts  of  stroma-tissue  separate  the  papillary  ducts.  Not  only  the  blood-vessels,  but 
also  the  nerve-trunks  are  pro\ided  with  sheaths  of  renal  stroma. 

The  blood-vessels  supplying  the  kidney,  branches  of  the  renal  artery, 
enter  the  renal  substance  through  the  vascular  foramina  surrounding  the 
papillae  on  the  wall  of  the  sinus.  As  they  pass  along  the  sides  of  the  papillae, 
their  positions  correspond  to  the  primary  interlobar  tracts  of  connective 
tissue  of  the  foetal  kidney.  On  reaching  the  level  of  the  bases  of  the  renal 
p^•ramids,  each  interlobar  artery  breaks  up  into  twigs,  some  of  which  pursue 
an  irregularly  arched  course  across  the  bases  of  the  pyramids,  thereby  pro- 
ckicing  in  places  the  impression  of  "arcades"  at  the  junction  of  the  medulla 
and  cortex.  From  these  arcuate  arteries  or  their  divisions  arise  the  terminal 
branches  which  supply  the  cortex.  The  cortical  twigs  radiate  generally  per- 
pendicular to  the  free  surface,  towards  which,  as  the  interlobular  arterioles, 
they  pass,  giving  off  the  short  lateral  twigs  that  end  in  the  vasa  afferentia 
of  the  glomeruli.  These  are  arranged  in  columnar  groups  along  the  path  of 
the  interlobular  cortical  arterioles.  Some  of  the  latter  continue  to  the  free 
surface  where,  in  conjunction  with  direct  capsular  branches  from  the  renal 
artery,  they  supply  the  capsule  of  the  kidney.  After  traversing  the  capillary 
complex  of  the  glomerulus,  the  blood  is  carried  off  by  the  vas  efferens,  which 
on  its  exit  immediately  resolves  into  the  cortical  capillaries,  whose  meshes 
about  the  convoluted  tubules  are  round  and  about  the  tubules  of  the  medul- 
lary rays  are  elongated.  In  addition  to  the  usual  path  through  the  glomer- 
ulus, the  kidney  substance  is  supplied  also  by  terminal  vessels  that  pass 
directly  into  the  intertubular  network.  The  course  of  the  medullary  twigs 
is  influenced  by  the  radial  disposition  of  the  tubules  between  which  they  run ; 
they  are,  therefore,  relatively  straight  and,  hence,  known  as  arteriolcc  rectce. 
They  arise  only  exceptionally  from  the  arcuate  arteries,  and  chiefly  from  the 
afferent  branches  of  low  lying,  perhaps  atrophic  (Huber),  Malpighian 
bodies.  The  supply  of  the  medulla  is,  therefore,  less  independent  than 
formerly  belie\'ed. 

The  veins  of  the  kidney  are  also  disposed  as  cortical  and  medullary 
branches,  which  empty  into  larger  stems,  the  so-called  vence  arciformes,  that 


214 


NORMAL   HISTOLOGY. 


cross  the  bases  of  the  pyramids  and  become  tributary  to  the  large  intralobar 
trunks.  The  blood  within  the  cortical  capillaries  escapes  by  three  paths: 
( I )  through  small  veins  that  pass  from  the  outer  third  of  the  cortex  towards 
the  capsule,  beneath  which  they  empty  into  stems  running  parallel  to  the 


Glomerulus 
Malpighian  body 


Interlobular  artery 


Interlobular  vein 


Vasae  rectae  of 
medulla 


Stellate  vein 


Capillary  network 
in  labyrinth 


I  A- -^    Capillary  network 
f25j~'in  medullary  ray 


Large  blood- 
vessels at  junction 
of  cortex  and 
medulla 


Fig.  266.— Longitudinal  section  of  injected  kidney  of  dog,  showing  general  arrangement  of  blood-vessels 
of  cortex  and  adjoining  medulla.    X  40. 

free  surface  of  the  kidney;  from  three  to  five  of  these  horizontal  vessels  con- 
verge to  form  a  star-like  channel,  the  vena  stellata,  which  is  the  beginning  of 
the  interlobular  vein  that  passes  through  the  cortex  with  the  corresponding 
arteriole  to  the  arcuate  veins  at  the  base  of  the  pyramid;  (2)  through 
small  veins  that  empty  directly  into  the  interlobular  veins  at  various  levels; 


THE   RENAL   DUCTS.  215 

(3)  throug-h  the  deep  cortical  veins  that  traverse  the  inner  third  of  the  cortex 
and  are  direct  tributaries  of  the  venae  arciformes.  The  medulla  is  drained  by 
the  venulae  rectae,  straight  vessels  that  begin  in  the  medullary  capillary  net- 
work and  empty,  for  the  most  part  indirectly,  in  the  arcuate  veins.  The 
latter  terminate  in  the  larger  interlobar  veins,  which  accompany  the  corre- 
sponding arteries  along  the  sides  of  the  pyramids  and  emerge  into  the  sinus 
around  the  papillae  to  become  tributary  to  the  renal  vein.  This  vessel  and 
its  branches  are  without  \'alves. 

The  lymphatics  of  the  kidney  occur  as  deeper  and  superficial  networks. 
The  deep  lymphatics  arise  as  networks  of  capillaries  within  the  cortex  and 
medulla,  the  general  path  of  the  more  definite  lymph-channels  being  that  of 
the  blood-vessels,  from  four  to  seven  large  trunks  emerging  at  the  hilum. 
The  superficial  lymphatics  include  two  networks,  one  situated  within  or 
beneath  the  fibrous  capsule  and  the  other  within  the  perirenal  fatty  areolar 
tissue  {capsula  adiposa).  The  subcapsular  channels  communicate  with  the 
peripheral  parts  of  the  cortical  network,  as  well  as  with  the  vessels  outside 
the  fibrous  capsule. 

The  nerves  supplying  the  kidney  are  derived  from  the  renal  sympathetic 
plexus  and  consist,  therefore,  principally  of  nonmeduUated  fibres.  These 
accompany  the  blood-vessels,  around  which  they  form  plexuses  containing 
ganglion-cells,  and  to  which  they  send  filaments  for  the  walls.  The  smaller 
arteries  are  accompanied  by  the  nerves  as  far  as  the  glomeruli  and  capillaries. 
The  relation  between  the  nerve-fibres  and  the  tubules  is  intimate,  delicate 
filaments  enclosing  the  convoluted  canals  with  a  network  outside  the  base- 
ment membrane  (epilemmar  plexus),  from  which  filaments  pass  to  the  inner 
side  of  the  membrane  (hypolemmar)  and  end  partly  between  the  epithelial 
cells. 

THE   RENAL   DUCTS. 

Each  canal  consists  of  the  greatly  expanded  upper  end,  the  renal  pelvis 
with  its  subdi\isions,  the  calyces,  and  the  main  part  of  the  duct,  the  ureter, 
the  whole  serving  for  the  collection  of  the  urine  as  it  escapes  from  the  kidney 
and  its  transmission  to  the  bladder. 

The  wall  of  all  parts  of  the  renal  duct  is  the  same  in  its  general  structure 
and  consists  of  three  layers:  (i)  the  mucoics  membrane,  (2)  the  viuscular 
tunic,  and  (3)  Xh^Jibrous  coat;  the  mucous  and  muscular  layers  are  more  or 
less  blended,  so  that  a  distinct  submucosa  is  wanting.  The  mucous  mem- 
brane is  clothed  with  transitional  epithelium  consisting  of  several  strata  of 
cells,  the  deepest  elements  being  irregularly  columnar  and  the  superficial 
ones  to  a  varying  degree  flattened.  The  tunica  propria  is  made  up  of 
bundles  of  fibrous  tissue,  intermingled  with  comparatively  few  elastic  fibres, 
and  is  often  directly  attached  to  the  muscular  coat,  a  meagre  amount  of 
loose  fibro-elastic  tissue  in  places  suggesting  a  submucous  layer.  Within 
the  ureter,  the  mucous  membrane  is  usually  thrown  into  longitudinal  folds 
and,  hence,  the  lumen  appears  stellate  in  transverse  sections.  Neither  well 
marked  papillae  nor  true  glands  are  present,  although  in  places  the  tunica 
propria  encroaches  upon  the  epithelium  and  subdivides  the  latter  into  nest- 
like groups  of  cells.  Numerous  scattered  colorless  blood-cells  are  ordinarily 
encountered  within  the  tunica  propria;  sometimes,  particularly  in  the  vicinity 
of  the  calyces,  they  are  aggregated  into  distinct  minute  lymph-nodules. 
On  the  papillae,  the  epithelium  lining  the  calyces  is  continuous  with  that  of 
the  papillary  canals,  while  the  tunica  propria  blends  with  the  intertubular 
renad  stroma. 


2l6 


NORMAL    HISTOLOGY. 


The  muscular  tunic  consists  of  loosely  connected  bundles  of  unstriped 
tissue  whose  continuity  as  definite  sheets  is  interrupted  by  often  considerable 

intervening-  fibrous  tissue. 
The  muscle-bundles  are 
arranged  as  a  thin  and  im- 
perfect longitudinal  and 
a  thicker  and  chief  exter- 
nal circular  layer.  With- 
in the  renal  pelvis  and  its 
larger  subdivisions,  the 
infundibula,  both  layers 
are  well  represented,  but 
are  reduced  on  the  caly- 
ces, except  at  the  junction 
of  the  latter  with  the  renal 
papillae  where  the  circular 
muscle  is  augmented  and 
surrounds  each  papilla 
with  a  minute  sphincter- 
like bundle.  In  the  lower 
half  of  the  ureter,  an  addi- 
tional but  incomplete  ex-, 
ternal  longitudinal  layer 
is  found  outside  the  cir- 
cular one.  At  its  end, 
where  the  ureter  meets 
and  traverses  the  wall 
of  the  bladder,  the  mus- 

FiG.  267. — Longitudinalsection  through  sinus  of  child's  kidney,  show-     Cular  tisSUe  is  represented 
ing  lower  part  of  pelvis  and  commencement  of  ureter.    X  10.  1  ^  1       •       1        1 

almost    exclusively   by  a 
well  developed  layer  of  longitudinal  fibres,  which  retain  their  independence 


Mucous  coat 
thrown  into 
longitudinal  folds     ^ 


k''v>; 


Ureter 


V 


_    >'-~^  Circular  muscular 
■^''^' ''  bundles 


Fig.  268. — Transverse  section  of  ureter.    X  25. 

and  do  not  blend  with  the  vesical  muscle  but  end  in  the  mucosa  of  the 
bladder.      Contraction  of  these  fibres  tends  to  dilate  the  ureteral  orifices. 


THE   BLADDER.  217 

The  fibrous  coat,  or  tunica  adventitia,  composed  of  bundles  of  fibro- 
elastic  tissue,  invests  the  entire  renal  duct  as  its  outermost  tunic  and  connects 
it,  through  the  surrounding  areolar  tissue,  with  the  adjacent  structures. 
Within  tiie  sinus  of  the  kidney,  the  outer  coat  of  the  renal  duct  blends  with 
the  tunica  fibrosa  that  in\ests  the  renal  substance  where  the  latter  is  not 
embraced  within  the  calyces.  Just  abo\-e  the  bladder  the  fibrous  coat  of  the 
ureter  becomes  thicker  and,  in  conjunction  with  the  longitudinal  muscle, 
f(^rms  the  tireteral  sheath,  gi\'ing  independence  to  the  duct  as  it  passes 
through  the  wall  of  the  bladder. 

The  blood-vessels  supplying  the  renal  duct,  derived  from  several 
sources  during  its  long  course  through  the  abdomen  and  pelvis,  break  up 
into  capillaries  which  are  especially  numerous  within  the  tunica  propria 
immediately  beneath  the  epithelium.  The  veins  begin  within  the  mucosa. 
l)eneath  which  they  form  an  inner  plexus  that  communicates  with  a  wider 
meshed  outer  plexus,  lying  within  the  fibrous  coat  and  giving  rise  to  the 
larger  emergent  venous  trunks.  The  lymphatics  within  the  mucous  mem- 
brane are  indefinite  lymph-channels,  but  within  the  muscular  coat  and  on  the 
surface  are  present  as  distinct  networks,  from  which  afferent  vessels  pass  to 
various  groups  of  lymph-nodes.  The  nerves  distributed  to  the  renal  duct 
are  brought  bv  branches  from  the  neighboring  sympathetic  plexuses.  While 
consisting  chiefly  of  nonmedullated  fibres  destined  for  the  muscular  tissue 
and  blood-vessels,  many  sensory  fibres  find  their  way  into  the  mucous  mem- 
brane, where  some  end  within  the  tunica  propria  in  free  arborescent  endings 
and  others  between  the  epithelial  cells. 

THE    BLADDER. 

The  bladder,  the  reservoir  in  which  the  urine  is  received  from  the  renal 
ducts  and  retained  until  discharged  through  the  urethra,  is  essentially  a 
muscular  sac,  lined  with  mucous  membrane  and  partly  covered  with  perito- 
neum, a  layer  of  connective  tissue  loosely  uniting  the  mucous  and  muscular 
coats.  From  within  outwards,  four  coats  are  distinguishable — the  nuicous, 
the  submucous,  the  muscular  and  th.^  fibrous. 

The  mucous  coat  closely  resembles  that  of  the  renal  duct,  consisting 
of  a  fibro-elastic  tunica  propria  covered  by  transitional  epithelium.  The 
details  of  the  latter  are  materially  affected  by  the  degree  of  contraction  or 
distention  to  which  the  mucosa  is  subjected,  since  it  is  probable  that  the 
impression  of  a  many-layered  epithelium  is  based  on  the  examination  of  sec- 
tions of  the  strongly  contracted  organ.  As  ordinarily  seen,  the  deepest  cells 
are  irregularly  columnar,  the  ones  of  the  middle  layers  polyhedral  or  club- 
shaped,  and  the  surface  cells  somewhat  flattened,  with  their  deeper  aspect 
modelled  by  the  subjacent  elements,  over  and  between  which  fit  depressions 
and  projections.  Although  definite  glands  can  hardly  be  said  to  exist,  in 
the  vicinity  of  the  vesical  trigone  and  of  the  urethral  orifice  the  tunica 
propria  contains  small  epithelial  pockets  or  crypts,  from  the  bottom  of  which 
short  branched  tubules,  fined  with  low  columnar  cells,  extend  into  the  sur- 
rounding stroma.  These  rudimentary  glands  have  been  interpreted  as 
representing  abortive  prostatic  tubules,  which  have  become  displaced  during 
the  development  of  the  lower  segment  of  the  bladder  from  the  uro-genital  sinus. 

The  submucous  coat,  loose  and  elastic,  permits  free  gliding  of  the 
mucous  membrane  over  the  muscular  tunic  when  readjustment  becomes 
necessary  during  contraction  and  the  mucosa  is  strongly  wrinkled.  It  is 
composed  of   bundles  of    fibrous  tissue,  interwoven   with  numerous  elastic 


2l8 


NORMAL   HISTOLOGY. 


fibres,  and  supports  the  blood-vessels  and  nerve-plexuses,  and  occasionally 
contains  small  lymph-nodules.  The  submucosa  is  not  sharply  defined  from 
the  adjacent  coats,  but  blends  with  the  tunica  propria  on  the  one  side  and 
penetrates  between  the  tracts  of  muscle-bundles  on  the  other.  Beneath  the 
trigonum  a  distinct  submucous  layer  is  wanting,  or  replaced  by  a  sheet  of 
muscular  tissue. 

The  muscular  coat,  thicker  than  the  mucous  and  comparatively 
robust,  varies  according  to  the  condition  of  the  bladder,  being  thin  during 
distention  and  very  thick  during  strong  contraction,  when  it  may  measure 
as  much  as  1.5  cm.  The  bundles  of  involuntary  muscle  are  arranged  as  two 
fairly  distinct  chief  layers — a  thick  circular  and  a  thin  outer  longitudinal. 
Inside  the  circular,  virtually  within  the  mucosa,  lies  an  incomplete  additional 


Epithelium 


Obliquely  cut 
longitudinal 
bundles 


Fibrous  coat 


Fig.  269. — Section  of  wall  of  bladder,  showing  general  disposition  of  coats.    X  12. 

layer  of  mostly  oblique  bundles.  The  longitudinal  bundles^  best  developed 
on  the  upper  and  lower  surfaces  of  the  bladder,  do  not  form  a  continuous 
sheet  but  interlace,  leaving  intervals  occupied  by  connective  tissue.  The 
circular  layer,  although  more  robust  and  uniform  than  the  outer  one,  is 
weak  and  imperfect  over  the  trigonal  region  and  is  well  developed  only 
above  the  level  of  the  orifices  of  the  ureters,  towards  the  apex  of  the 
bladder  becoming  oblique  and  less  regular.  The  innermost  layer,  largely 
represented  by  isolated  and  indefinite  bundles  intermingled  with  connective 
tissue,  is  condensed  over  the  trigone,  where  it  exists  as  a  compact  muscular 
sheet  closely  united  with  the  overlying  mucous  membrane,  and  surrounds 
the  orifices  of  the  ureters  and  of  the  urethra  with  sphincter-like  bands. 

The  fibrous  coat,  composed  of  fibro-elastic  bundles,  is  blended  over 
the  upper  and  lateral  surfaces  of  the  bladder  with  the  serous  (peritoneal) 
covering;  where  this  is  wanting,  it  is  continuous  with  the  areolar  tissue  con- 
necting the  bladder  with  the  surrounding  pelvic  wall  and  organs.  It  is 
strongest  over  the  inferior  surface,  where  it  receives  additions  from  the 
pelvic  fascia,  while  towards  the  apex  and  beneath  the  peritoneum  it  is  less 
definite  and  intermingled  with  adipose  tissue. 


THE   URETHRA.  219 

The  blood-vessels  enclose  the  bladder  with  an  arterial  network  within 
the  fibrous  coat,  from  which  twigs  enter  the  muscular  coat  and  break  up  into 
capillaries,  while  others  gain  the  submucous  layer  and  form  a  network  of  the 
larger  stems;  from  this  branches  pass  into  the  mucous  membrane  and  give 
rise  to  a  rich  capillary  meshwork  immediately  beneath  the  epithelium.  The 
veins  form  a  submucous  plexus  that  drains  the  mucosa  and  empties  into  a 
muscular  plexus  which,  in  turn,  is  tributary  to  the  external  subperitoneal 
plexus.  With  the  exception  of  the  smaller  ones  on  the  inferior  surface,  the 
vesical  veins  possess  valves.  The  lymphatics  begin  as  a  close-meshed 
plexus  within  the  muscular  coat,  distinct  lymph-channels  being  absent  within 
the  mucous  membrane.  Outside  the  muscular  coat  they  form  a  loose  plexus 
within  the  fibrous  tissue  (subperitoneal),  the  lymphatics  from  the  apex  and 
body  of  the  bladder  coursing  downwards  and  laterally  and  those  from  the 
fundus  upwards. 

The  nerves  include  both  spinal  and  sympathetic  fibres,  medullated  and 
nonmedullated,  and  within  the  fibrous  coat  are  connected  with  ganglia,  par- 
ticularly in  the  vicinity  of  the  ureters,  from  which  twigs  enter  the  muscular 
coat  and  break  up  into  smaller  ones  bearing  microscopic  ganglia.  Other 
branches  gain  the  submucous  layer,  where  they  form  plexiform  enlarge- 
ments, containing  numerovis  groups  of  ganglion-cells;  in  addition  to  sympa- 
thetic filaments  to  the  blood-vessels,  fine  bundles  of  fibres  proceed  to  the 
mucous  membrane  to  end  in  free  terminations  partly  within  the  tunica 
propria  and,  probably,  to  some  extent  between  the  epithelial  cells.  The 
general  sensibility  of  the  normal  bladder  is  comparatively  slight,  being 
greatest  in  the  trigonal  region,  especially  at  the  ureteral  openings. 

THE  URETHRA. 

The  urethra — the  canal  through  which  the  urine  is  conveyed  from  the 
bladder  to  the  exterior  of  the  body — differs  in  the  two  sexes,  since  in  the 
male,  in  addition  to  its  common  function  of  conducting  the  urine,  it  serves 
for  the  escape  of  the  secretions  of  the  sexual  glands. 

The  Male  Urethra. — Considered  with  regard  to  the  regions  of  the 
body  in  which  it  lies,  the  male  urethra  may  be  divided  into  a  pelvic,  a  peri- 
neal and  a  penile  portion.  It  is  more  usual,  however,  to  describe  the  canal 
as  consisting  of  the  prostatic,  the  membranous  and  the  spongy  portions — a 
division  based  on  the  anatomical  relations  to  the  structures  through  which 
it  passes. 

The  wall  of  the  urethra  consists  of  a  mucous  membrane  containing  a 
rich  venous  plexus  and  supplemented,  in  the  prostatic  and  membranous 
portions,  by  considerable  tracts  of  muscular  tissue.  The  tunica  propria 
possesses  an  unusual  number  of  fine  elastic  fibres  and  is  covered  with  an 
epithelium  that  varies  in  different  parts  of  the  canal.  Throughout  the  upper 
two-thirds  of  the  prostatic  portion,  the  epithelium  resembles  that  of  the 
bladder,  being  of  the  transitional  variety;  on  approaching  the  membranous 
portion,  the  epithelium  becomes  stratified  columnar  in  type,  small  reserve 
cells  lying  between  the  outer  ends  of  the  superficial  elements,  which  are 
often  goblet-cells.  Except  within  the  beginning  of  the  spongy  portion  (pars 
cavernosa),  where  the  lining  is  reduced  to  practically  a  single  stratum  of 
cells,  the  columnar  epithelium  retains  its  stratified  character  as  far  as  the 
navicular  fossa,  the  dilated  distal  end  of  the  urethra.  Here  the  epithelium 
becomes  stratified  squamous  and  at  the  external  urethral  orifice  is  directly 
continuous    with    the    epidermis    covering    the   glans    penis.     Within    the 


NORMAL    HISTOLOGY. 


\ 


urethral  crest,  the  fusiform  ridge  modeUing  the  posterior  wall  of  the  prostatic 
portion  of  the  canal,  the  mucous  membrane  acquires  the  character  of  erectile 
tissue  on  account  of  the  abundance  of  the  venous  channels  occupying  the 
deeper  la3'er  of  the  tunica  propria. 

The  muscular  tissue  associated  with  the  male  urethra  includes  in- 
trinsic and  extrinsic  fibres,  the  former  being  involuntary  and  directly  incor- 
porated with  the  wall  of  the  canal  and  the  latter  being  accessory  strands  of 
striped  muscle  derived  from  structures  surrounding  the  urethra.  The  in- 
trinsic muscle  is  arranged  as  an  inner  longitudinal  and  an  outer  circular 
layer,  of  which  the  longitudinal  is  thinner  but  more  widely  distributed, 
extending  from  the  internal  urethral  orifice  at  the  bladder  as  far  forwards 
as  the  openings  of  the  bulbo-urethral  glands  in  the  pars  cavernosa.  The 
circular  fibres,  outside  the  longitudinal  ones,  are  best  developed  at  the 
internal  urethral  orifice,  where  they  are  three  or  four  times  as  thick  as  those 
running  lengthwise.      They  gradually  diminish  and  just  beyond  the  mem- 


Surface  epithelium 


Intramucous  glands 


m^^  ,  ,:>p  %t  •'^^^-n^m. 


Tunica 
propria 


Fig.  270. — Section  of  mucous  membrane  of  prostatic  urethra,  showing  gland-like  crypts  in  mucosa.    X  45. 

branous  urethra  disappear,  first  on  the  lower  and  last  on  the  upper  wall  of 
the  dilatation,  the  fossa  bulbi.  In  advance  of  the  posterior  third  of  the 
spongy  portion,  the  intrinsic  muscle  is  wanting,  the  unstriped  muscular 
tissue  surrounding  the  remainder  of  the  canal  belonging  to  the  erectile  tissue 
of  the  corpus  spongiosum  which  the  urethra  traverses.  The  iiiternal  vesical 
sphincter,  encircling  the  commencement  of  the  urethra,  is  derived  from  the 
muscular  sheet  of  the  trigonum  of  the  bladder.  At  the  apex  of  the  prostate 
gland,  the  urethra  is  surrounded  by  bundles  of  striped  muscle,  known  as  the 
external  vesical  sphincter,  prolonged  upwards  from  the  compressor  urethrae 
muscle. 

The  urethral  glands,  or  glands  of  Littrc,  embrace  two  groups,  those 
within  the  mucous  membrane  and  those  immediately  outside,  whose  ducts 
are  seen  with  a  magnifying  glass  as  minute  openings  on  the  mucous  surface. 
The  former,  the  intramucous  glands,  are  small  and  simple,  consisting  usually 
of  a  single  alveolus,  less  frequently  of  two  or  three.  They  are  lined  with 
columnar  epithelium  and  occur  in  all  parts  of  the  urethra,  but  are  most 
numerous  in  the  spongy  portion.  The  extramiicons  glands,  although  small, 
are  larger  than  the  preceding  but  less  widely  distributed,  since  they  are 
absent  in  the  distal  half  of  the  membranous  and  the  proximal  third  of  the 


THE    URETHRA.  221 

spongy  portion.  They  are  most  abundant  and  best  developed  in  the  upper 
wall  of  the  spongy  portion  distal  to  the  openings  of  the  ducts  of  the  bulbo- 
urethral (Cowper'sj  glands.  Their  ducts  often  extend  several  millimeters 
obliquely  backwards,  more  or  less  parallel  to  the  urethra,  and  divide  into 
several  slightly  expanded  alveoli,  lined  with  columnar  epithelium.  In  addi- 
tion to  the  foregoing  true,  although  small  glands,  the  urethral  mucosa  is 
beset,  along  its  upper  wall,  with  small  diverticula,  the  laciiiuc  iirethralcs, 
which  are  little  more  than  tubular  depressions,  or  crypts,  within  the  mucous 
membrane  and  can  not  be  regarded  as  glands,  although  they  often  receive 
the  ducts  of   the   cxtramucous  glandules.      One  crypt   of   exceptional  size 


Surface  epithelium 


Crypt 


Blood-vessels 
in  mucosa 


Venous  spaces 
of  cavernous 
tissue 


Fig.  271. — Section  of  wall  of  urethra,  spongy  portion,  showing  crypts  and  blood-spaces  in  mucosa.      ■   35. 

(4-12  mm.  in  depth)  is  commonly  found  on  the  roof  of  the  navicular  fossa, 
its  orifice  being  guarded  by  a  fold  of  mucous  membrane. 

The  Female  Urethra. — The  wall  of  this  canal,  much  shorter  than 
the  urethra  in  the  male,  consists  essentially  of  a  mucous  membrane  supple- 
mented by  a  robust  outer  muscular  tunic.  The  mucous  membrane, 
thrown  into  longitudinal  folds,  is  composed  of  a  tunica  propria,  rich  in  elastic 
fibres,  wandering  cells  and  plexiform  veins,  and  a  layer  of  epithelium.  The 
latter  resembles  that  of  the  bladder  above  and  that  of  the  vestibule,  into 
which  the  canal  opens,  below,  but  in  places  the  stratifieci  squamous  epithe- 
lium gives  way  to  one  approaching  the  simple  columnar  in  type.  In  the 
female  the  urethral  glands  are  represented  by  small  tubular  alveoli  that  open 
by  minute  ducts  on  the  mucous  surface  and  correspond  to  Littre's  glands  in 
the  male.  They  are  most  plentiful  in  the  upper  part  of  the  urethra  and  in 
aged  subjects  may  contain  concretions.  The  mucosa  is  also  pitted  with 
small  crypts,  similar  to  those  in  the  male  canal,  into  which  the  ducts  of  the 
glands  often  open. 

The  intrinsic  muscular  tissue  of  the  female  urethra  is  disposed  as  an 
inner  layer  of  longitudinal  and  an  outer  one  of  circular  fibres,  the  two  being 
separated  by  a  thin  connective  tissue  stratum  with  many  elastic  fibres.  At 
the  internal  urethral  orifice,  in  conjunction  with  fibres  from  the  trigonum  of 


222  NORMAL   HISTOLOGY. 

the  bladder,  the  circular  fibres  aid  in  forming  the  internal  vesical  sphincter. 
The  lower  end  of  the  urethra  is  embraced  by  the  anterior  fibres  of  the 
sphincter  vaginae  muscle,  and  higher,  between  the  layers  of  the  triangular 
ligament,  the  canal  is  surrounded  by  the  bundles  of  the  compressor  urethrae. 
The  deepest  part  of  the  mucosa  and  the  adjacent  portion  of  the  longitudinal 


Longitudinal 
muscle 


Circular- 
muscle 

Fig.  272. — Longitudinal  section  of  wall  of  female  urethra.     X  50. 

muscle  layer  in  places  contain  such  a  rich  venous  plexus  that  the  tissue 
resembles  a  cavernous  structure. 

The  blood-vessels  supplying  the  urethra  provide  a  generous  capillary 
network  beneath  the  epithelium ;  it  is,  however,  the  unusual  abundance  of 
the  venous  channels  that  confers  the  exceptional  cavernous  character  to  the 
wall  of  the  canal.  The  lymphatics  are  also  numerous  within  the  'deeper 
parts  of  the  mucosa  in  the  male,  especially  in  the  region  of  the  glans  penis; 
towards  the  upper  end  of  the  urethra  they  diminish,  those  from  the  prostatic 
portion  communicating  with  the  intramuscular  network  at  the  neck  of  the 
bladder.  The  lymphatics  of  the  female  urethra  correspond  with  those  of 
the  membranous  and  prostatic  portions  of  the  male  duct.  The  nerves 
include  branches  from  the  pudic  (sensory  fibres  to  the  mucous  membrane 
and  motor  fibres  to  the  associated  striped  muscle)  and  from  the  hypogastric 
plexus  of  the  sympathetic  by  way  of  the  prostatic  and  cavernous  plexus. 
The  plexiform  sympathetic  fibres,  associated  with  numerous  ganglion-cells 
along  their  course,  supply  the  involuntary  muscle  and  the  walls  of  the  blood- 
vessels. The  sensory  fibres  are  distributed  to  the  mucous  membrane  in 
which  they  end  mostly  as  free,  but  to  some  extent  as  special  terminations 
within  the  tunica  propria,  although  some  filaments  penetrate  between  the 
epithelial  cells. 

THE   SUPRARENAL   BODIES. 

These  are  a  pair  of  cocked-hat-shaped  organs,  about  6. 5  cm.  long  and 
half  as  broad,  situated  at  the  back  of  the  abdominal  cavity,  on  the  inner 
aspect  of  the  upper  ends  of  the  kidney.  This  proximity  suggests,  as  a 
matter  of  convenience,  the  description  of  the  suprarenal  body  in  connection 
with  the  urinary  organs;  it  must  be  understood,  however,  that  the  supra- 
renal has  neither  morphological  nor  functional  relations  to  the  kidney  which 
warrant  such  association. 


THE   SUPRARENAL    BODIES. 


223 


,  &.  Capsule 


Cortex 


The  suprarenal  body  is  invested  by  a  distinct  fibrous  raps?(/e,  from  which 
deHcate  septa  pass  into  the  organ,  forming  a  framework  of  connective  tissue 
for  the  support  of  the  blood-vessels  and  the  cellular  constituents.  Section 
across  the  thicker  parts  of  the  body  displays  an  outer  zone,  the  corfex,  from 
.25-1.25  mm.  in  thickness,  which  encloses  a  central  area,  the  77iedulla. 
Towards  the  borders  of  the  organ,  the  medulla  is  reduced  to  a  narrow  zone, 
or  may  be  entirely  wanting;  where  best  developed,  as  in  the  middle,  it  may 
attain  a  thickness  of  over  3  mm.  The  cortex  is  usually  of  a  dirty  yellow  color, 
next  the  medulla  presenting  a 
narrow  band  of  varying  shades 
of  brown.  The  medulla  is  of 
a  grayish  tint  and  generally 
lighter  than  the  cortex.  Its 
exact  color,  however,  varies 
with  the  amount  and  condition 
of  the  contained  blood,  when 
engorged  with  venous  blood 
being  dark.  Embryology  and 
comparati\-e  anatomy  indicate 
that  the  mammalian  supra- 
renal body  inciudes  two  en- 
tirely distinct  organs,  which, 
although  intimately  united 
as  the  cortex  and  medulla, 
possess  different  origins  and 
functions.  The  cortex  arises 
in  close  relation  with  the 
Wolffian  body,  the  fcetal  ex- 
cretory gland,  and  later 
migrates  into  secondary  rela- 
tion with  the  kidney.  The 
medulla  is  derived  from  the 
adjacent  embryonic  sympa- 
thetic ganglia,  the  medullary 
cells  closely  corresponding 
with  the  chromafifin  elements 
elsewhere  originating  from 
the  sympathetic. 

The  cortical  substance 
consists  of  a  delicate  frame- 
work of  connective  tissue,  prolonged  from  the  capsule,  in  whose  meshes  lie 
the  tracts  of  the  distinctive  epithelial  cells.  The  cortex  is  not  always  uniform, 
but  often  subdivided  into  indistinct  subcapsular  lobules  by  thicker  septa  con- 
tinued from  the  overlying  fibrous  investment.  The  arrangement  of  the  cortical 
cells,  although  in  a  general  way  columnar  or  slightly  radial  in  the  peripheral 
lobular  areas,  varies  at  different  levels,  three  zones  being  distinguished  within 
the  cortex  (Fig.  274).  The  narrow  zona glomerulosa  lies  next  the  capsule  and 
consists  of  rounded  masses  of  somewhat  tortuous  groups  of  cells.  The  zona 
fasciculata  forms  the  major  part  of  the  cortex  and  is  made  up  of  parallel  or 
slightly  radially  disposed  cell-columns.  The  zo7ia  reticularis,  next  the  medulla 
and  narrow,  includes  the  networks  of  epithelial  elements  formed  by  the  union 
of  the  inner  ends  of  the  columns  of  the  preceding  zone.  The  cells  through- 
out the  cortical  strands  are  fairly  similar,  being  rounded  polygonal  elements, 


Fig.  273. — Section  of  suprarenal  body  including  entire  thick- 
ness of  the  organ,  showing  the  general  arrangement  of  cortex 
and  medulla.     X  27. 


224 


NORMAL    HISTOLOGY. 


Capsule 


15-20  ;/  in  diameter,  whose  cytoplasm  lodges  a  variable,  but  usually  large 
number  of  fat  granules.  The  latter  are  very  abundant  in  the  cells  of  the 
zona  fasciculata,  but  few  or  entirely  absent  in  those  of  the  zona  reticularis. 
Within  the  last  named  region,  however,  the  cells  are  more  or  less  pigmented, 
a  peculiarity  accounting  for  the  darker  tint  of  this  part  of  the  cortex  fFig. 

___ _^____  274).      The    cells    composing 

"  '         ~  the    cords    and    columns   are 

in  direct  contact  with  one 
another;  neither  are  they,  as 
groups,  surrounded  by  a  base- 
ment membrane,  but  come 
into  close  relation  with  the 
capillary  blood-vessels  that 
enclose  the  cell-islands,  a  few 
delicate  strands  of  supporting 
tissue  alone  intervening. 

The  medullary  sub- 
stance consists  chiefly  of  net- 
works of  anastomosing  cords 
of  polyhedral  cells  (20-30  //  in 
diameter),  so  prone  to  post- 
mortem change  that,  as  usu- 
ally seen,  they  are  very  irreg- 
ular and  often  stellate.  They 
are  distinguished  from  the  cor- 
tical elements  by  the  affinity 
of  their  cytoplasm  for  chromic 
acid  and  its  salts,  staining 
yellow  or  brown.  They  are 
known,  therefore,  as  chromaf- 
fin cells ^  and  regarded  as  akin 
to  sympathetic  elements.  In 
addition  to  these  cells,  the  me- 
dulla contains  numerous  blood- 
vessels, particularly  venous 
channels,  and  many  bundles 
of  nerve-fibres  and  ganglion- 
cells.  The  latter  (Fig.  275; 
occur  singly  or  in  small  groups, 
are  multipolar  or  stellate,  and 
resemble  the  cell-bodies  of 
sympathetic  neurones. 

The  blood-vessels  sup- 
plying the  suprarenal  body 
divide  into  a  dozen  or  more 
fine  branches  which  enter  by 
piercing  the  capsule  at  various  points,  some  penetrating  directly  as  far  as  the 
medulla,  but  most  of  them  terminating  within  the  cortex.  These  last  form 
a  superficial  network  from  which  the  capillaries  extend  between  the  cell- 
cords,  which  are  thus  enclosed  within  vascular  meshes  of  corresponding  form. 
Thus,  within  the  cortex  the  meshes  are  elongated  and  within  the  medulla  of 
rounded  form.  In  large  part  the  medulla  is  supplied  by  arterioles  passing 
directly  to  the  interior  of  the  organ.      These  break  up  into  capillaries  which 


Medulla 


274.  —  Section  of  suprarenal  body,  showing  details  of 
superficial  and  deep  ponions  of  cortex.     X  225. 


THE   SUPRARENAL    BODIES. 


225 


surround  the  medullary  cords  and,  in  conjunction  with  the  capillaries  of  the 
deeper  part  of  the  cortex,  pass  over  into  an  unusually  rich  plexus  of  veins 
terminating  in  the  large  central  vessel,  the  beginning  of  the  chief  suprarenal 
vein.  Superficial  veins  claim  as  tributaries  the  peripheral  portions  of  the 
capillary  network. 

The  lymphatics  are  represented  by  a  network  within  the  zona  glomer- 
ulosa  which  communicates  with  the  subcapsular  plexus,  on  the  one  hand, 
and,  by  means  of  centrally  directed  stems,  with  the  rich  medullary  plexus  on 
the  other.      The  larger  trunks  from  the  medullary  network  follow  the  veins 


r/ 


—^■^ —  Nerve-fibres 


I 


.Cords  of  medullary 
cells 


Blood-vessel 


-  Nerve-cell 


-Wall  of  vein 


-'O   "Kd 


Fig.  275. — Portion  of  medulla  of  suprarenal  body,  from  vicinity  of  central  vein.     X  280. 

and  emerge  at  the  hilum  of  the  organ  as  several  efferents  that  pass  to  adja- 
cent lumbar  lymph-nodes. 

The  nerves  are  remarkable  for  their  abundance  and  derived  principally 
from  the  solar  and  renal  sympathetic  plexuses,  fibres  from  the  vagus  being 
included.  They  form  a  plexus  within  the  capsule  from  which  bundles  of 
chiefly  nonmeduUated  fibres  pierce  the  capsule,  along  with  the  arteries,  and 
give  of?  fibres  that  pass  between  the  cell-cords  of  the  zona  glomerulosa  and 
fasciculata  to  end  in  the  walls  of  the  blood-vessels  and  on  the  surface  of  the  cell- 
groups.  Other  branches  penetrate  to  the  zona  reticularis  to  form  a  still  closer 
plexus,  but  it  is  for  the  medulla  that  the  most  striking  abundance  is  provided. 
Here  the  numerous  nerves  join  into  plexuses  from  which  fibres  pass  to  the 
blood-vessels  and  cords  of  medullary  cells.  In  contrast  to  their  superficial 
relation  to  the  cortical  cell-groups,  within  the  medulla  the  fibrils  penetrate 
between  the  cells,  so  that  almost  each  of  the  latter  comes  into  direct  relation 
with  a  nerve-fibre.  Numerous  sympathetic  ganglion-cells,  isolated  or  in 
small  groups,  are  also  present. 

Accessory  Suprarenals. — These  are  usually  very  small,  rarely  sur- 
passing a  pea  in  size.  They  may  be  found  near  the  suprarenal  body,  in  the 
kidney,  in  the  liver,  in  the  solar  and  renal  plexuses,  or  beside  the  testis  or 
the  ovary.  The  accessory  suprarenal  situated  in  the  broad  ligament  of  the 
15 


226 


NORMAL    HISTOLOGY. 


uterus,  in  the  vicinity  of  the  ovary,  is  known  as  Ma  re  hand '  s  organ,  and 
regarded  as  a  normal  and  ahiiost  constant  organ.  These  ' '  accessory ' '  bodies 
include  two  groups  of  different  origin  and  morphological  significance.      Those 

associated  in  position  with  the  chief  organ, 
as  when  in  the  liver  or  kidney,  are  derived 
from  separated  and  isolated  portions  of 
the  principal  embryonic  area  of  the  supra- 
renal and,  therefore,  are  supernumerary. 
The  bodies  situated  in  the  broad  ligament, 
or  in  relation  with  the  epididymis,  are, 
on  the  contrary,  probably  developed 
from  the  atrophic  tubules  of  the  Wolffian 
body  and,  hence,  must  be  regarded  as 
structures  independent  of  the  main  supra- 
renal. The  independent  bodies  never, 
and  the  supernumerary  ones  only  in  very 
exceptional  cases,  possess  more  than  corti- 
cal substance,  a  medulla  being  wanting. 

The  suprarenal  bodies  are  conceded 
membership  in  the  group  of  visce:ra  now 
termed  organs  of  intet'nal  secreiioti.  The 
other  members  of  this  group  are  the  thy- 
roid, the  parathyroids,  the  anterior  lobe  of 
the  pituitary  and,  probably,  the  thymus. 
The  specialized  areas  within  the  pancreas, 
the  islands  of  Langerhans,  are  also  re- 
garded as  producing  a  particular  sub- 
stance and,  hence,  are  minute  organs  of 
internal  secretion.  That  the  substances 
elaborated  within  these  viscera  are  of  im- 
portance is  evident  from  the  fact  that  re- 
moval of  either  the  suprarenals,  the  para- 
thyroids, or  the  pituitary  results  in  death. 
Although  at  present  the  role  of  the  cor- 
tical substance  of  the  suprarenal  is  unknown,  the  special  activity  of  its 
medulla  in  producing  adrenalin  seems  established. 


Fig.  276. — Section  of  injected  siiprarenal 
body  ;  the  vessels  in  the  lower  third  of  figure 
are  chiefly  tributary  to  the  central  vein.    X  25. 


THE  MALE  REPRODUCTIVE  ORGANS. 


These  include:  the  sexual  glands  (the  hs^cs),  the  spermatic  ducts  (ij>i- 
didymcs  and  vasa  defercntia)  and  their  appendages  (the  seminal  vesicles), 
the  copulative  organ  (the  penis),  and  certain  accessory  glands  (the  pros- 
tate and  the  bulbo-nrethral  glands).  At  first  situated  within  the  abdomen, 
the  testes  migrate  during  the  last  few  weeks  of  foetal  life  through  the  inguinal 
canal  into  the  scrotum,  gaining  the  latter  usually  shortly  before  birth.  In 
their  descent  they  are  accompanied  by  blood-\'essels,  lymphatics,  nerves  and 
their  ducts,  which  structures,  with  the  supporting  and  in\'esting  tissue,  con- 
stitute the  spermatic  cord  that  extends  through  the  abdominal  wall  to  the 
scrotum. 

THE   TESTIS. 

As  often  employed,  the  term  "testicle"  includes  two  essentially  differ- 
ent parts,  the  testis,  the  true  sexual  gland,  and  the  epididymis,  the  highly 
convoluted  commencement  of  the  spermatic  duct.  The  testes  or  testicles 
proper,  the  glands  producing  the  seminal  elements,  are  two  ellipsoidal  bodies 
obliquely  suspended  within  the  scro- 


Globus  major  of  epididymis 


-Vas  deferens 


turn.  Each  testis  measures  about 
4  cm.  in  length,  2.5  cm.  in  breadth 
and  2  cm.  in  thickness.  With  the 
exception  of  the  posterior  border, 
where  the  vessels,  nerves  and  ducts 
enter  and  emerge,  the  testis  is  covered 
with  a  serous  membrane,  the  tnnica 
vaginalis. 

Architecture  of  the  Testicle. 
— The  framework  of  the  testicle 
proper  includes  a  stout  fibro-elastic 
capsule,  the  tunica  albugi?iea,  .4— .6 
mm.  in  thickness,  that  gives  form  to 
the  organ  and  protects  the  enclosed 
soft  glandular  tissue.  Along  the  pos- 
terior border  of  the  testis  the  capsule 
is  greatly  thickened  and  projects  for- 
wards as  the  mediastinum  testis,  a 
wedge-shaped  body  (2. 5— 3  cm.  long) 
from  which  radiate  a  number  of 
membranous  septa  that  pass  to  the  inner  surface  of  the  tunica  albuginea. 
In  this  way  the  space  enclosed  by  the  capsule  is  subdi\'ided  into  pyramidal 
compartments  (Fig.  277),  the  bases  of  which  lie  at  the  periphery  and  the 
apices  at  the  mediastinum.  These  spaces  contain  collectively  from  150  to 
200  pyriform  masses  of  glandular  tissue,  more  or  less  separated  from  one 
another,  which  are  the  lobules.  Each  of  the  latter  is  made  up  of  from  one 
to  three  greatly  convoluted  seminiferous  tubules,  held  together  by  delicate 
vascular  intertubular  connective  tissue.  The  seminiferous  tubules,  from 
.15-25  mm.  in  diameter  and  from  25-70  cm.  (10-28  inches)  in  length, 
begin    as  blind    canals  ^\■hich  are  moderately  branched  and  very  tortuous 

227 


Rete  testis  in 
mediastinum 

.Tubiili 
contorti 

Vas  aberrans  — 

Ductus 
epididymidis 
Globus  minor 


Septum        Tunica  albuginea 


Fig.  277. — Diagram  illustrating  architecture  of  llie 
testicle. 


\ 


228 


NORMAL    HISTOLOGY. 


(tubuli  coiitorti)  throughout  their  course  until  tliey  converge  at  the  apex 
of  the  lobule.  Here  they  pass  over,  directly  or  after  union  with  another 
canal,  into  the  straight  tubules  {titbiili  recti)  that  enter  the  mediastinum 
and  join  to  form  a  close  network,  the  rete  testis.  The  latter  extends  almost 
the  entire  length  of  the  mediastinum  and  consists  of  a  system  of  irregular 


Epididymis 


Convolutions  of  duct  of  epididymis  in 
globus  major 


Coni  vasculosi 
(convolutions  of 
efferent  ducts) 


Efferent  ducts 


Digital  fossa  . 
Serous  surface  of  testis 


Testis 


.  Sections  of  duct 
of  epididymis 


-  — ^  Blood-vessels 


Rete  testis  in 
mediastinum 


Lobules  of  gland-tissue 


Interlobular  septum 


y 


Tunica  albuginea 


:Q^ 


"'"^m 


Convolutions  of  duct 
of  epididymis  in 
W0f^  globus  minor 


-^' 


7^ 


Fig.  278.— Longitudinal  section  of  testicle  of  child,  showing  arrangement  of  framework  and  gland-tissue 
and  of  canals  connecting  testis  with  epididymis.     X  10. 

intercommunicating  channels  lined  with  cuboidal  epithelium.  With  these 
passages  the  canals  of  the  testicle  proper  end,  the  immediate  continuation 
of  the  spermatic  path  being  from  15-20  tubules,  th&  dtiduli  effer-eiites,  that 
pierce  the  tunica  albuginea  along  the  posterior  border  of  the  testis  near  the 
upper  pole  and,  forming  the  progressively  tortuous  coni  vasculosis  connect 
the  sexual  gland  with  the  beginning  of  the  spermatic  duct,  the  highly  con- 
voluted dncttis  epididymidis . 


THE   TESTIS. 


229 


In  contrast  to  tlie  dense  fihro-elastic  tissue  of  the  framework  of  the  testis, 
the  interstitial  connective   tissue   between  the  seminiferous  tubules  is 


Blood-vessel 


Seminiferous  tubule,, 
cut  obliquely 


Tunica  albuginea 


Seminiferous  tubule, 
cut  transversely 


Group  of  interstitial  cells 


Tunica  vaginalis 


Fig.  279. — Transverse  section  of   testis,  showing  dense  fibrous  capsule  and  adjacent  seminiferous 

tubules.     X  30. 

loose,  consisting  of  delicate  bundles  of  white  fibrous  tissue  with  few  elastic 
fibres.  In  addition  to  plate-like  connective  tissue  cells,  leucocytes  and  eosin- 
ophiles  that  occur  in  varying 
numbers,  the  intertubular  or  in- 
terstitial stroma  contains  groups 
or  cord-like  masses  of  peculiar 
rounded  polygonal  elements,  the 
interstitial  cells.  These  cells 
(15-20  //  in  diameter)  possess 
relatively  small  eccentric  nuclei 
and  finely  granular  cytoplasm 
that  usually  lodges  numerous 
brownish  droplets,  pigment  par- 
ticles and,  often,  crystalloid 
bodies  in  the  form  of  minute 
needles  or  rods.  The  signifi- 
cance of  these  cells  is  uncertain, 
but  they  are  probably  modified 
connective  tissue  elements  de- 
rived from  the  mesoderm  of  the 
embr3^onic  germinal  ridge. 

The  Seminiferous  Tu- 
bules.— The  secreting  tubules 
consist  of  a  tunica  propria,  or 
basement  membrane,  which  en- 
closes several  layers  of  epitlie- 
lial  cells.  These  vary  not  only  before  and  after  the  attainment  of  se.xual 
maturit}',   but   subsequent   to    the   latter   with   functional   acti\ity  or   rest. 


Dilated  duct 


_^v^  Connecting 
canal 


Epithelium 


Blood-vessel 


2S0. — Section    of    mediastinum,    showing    irregular 
channels  of  rete  testis.     X  60. 


230 


NORMAL    HISTOLOGY. 


As  seen  in  sections  of  the  mature  human  testicle  (Fig.  281),  the  epithehal 
lining  of  the  seminiferous  tubules  includes  two  chief  varieties  of  cells,  the  sup- 
porting and  the  spennatogeneiic.  The  former,  the  Sertoli  cells,  take  no  active 
part  in  the  production  of  the  spermatozoa,  but  serve  as  temporary  supports 
for  t..e  more  essential  elements  during  certain  stages  of  spermatogenesis. 
They  are  elongated  irregularly  pyramidal  in  form  and  rest  by  expanded 
bases  upon  the  membrana  propria,  projecting  between  the  surrounding  sper- 
matogenetic  cells  towards  the  lumen  of  the  tubule.  Their  large  oval  nuclei 
are  meagre  in  chromatin  and  often  lie  at  some  distance  from  the  bases. 
The  outer  part  of  the  cytoplasm  contains  fat- droplets,  the  inner  zone  being- 


Tunica  propria 


Secondary  spermatoc>i;e  ^V^ 


Spermatids 


Spermatids  being 
transformed  into  spermatozoa 


Secondary  spermatocyte 
Spermatozoa 


Inner ^^«^Jr 

lamella  "^ 


Sertoli  cell 
Restmg  spermatogone 


Primarj  spermatoc>te 


Fig.  281. — Portion  of  a  seminiferous  tubule,  cut  transversely,  showing  the  lining  cells  in  various  stages  of 

spermatogenesis.    X  350. 

granular  or  longitudinally  striated.  Where  the  convoluted  tubules  pass  into 
the  straight  ones,  the  Sertoli  cells  become  reduced  in  height  and  form  a  layer 
of  simple  columnar  cells  continuous  with  the  low  cuboidal  epithelium  that 
lines  the  rete  testis  in  the  mediastinum. 

The  spermatogenetic  cells  are  concerned  in  the  cytological  cycle, 
known  as  sperniatoge7iesis,  whereby  the  spermatozoa  are  produced  from 
the  cells  lining  the  seminiferous  tubules.  They  include  four  forms  that 
stand  in  the  relation  of  succeeding  generations  to  one  another,  those  repre- 
senting the  oldest  lying  nearest  the  membrana  propria  and  the  youngest, 
from  which  the  spermatozoa  are  directly  derived,  next  the  lumen  of  the 
tubule. 


The  first  generation,  the  spermatogones,  lie  at  the  periphery  between  the  Sertoli 
cells  and,  although  small  round  elements,  possess  nuclei  exceedingly  rich  in  chromatin. 


SPERMATOGENESIS. 


231 


The  division  of  the  spermatogone  results  in  two  cells,  of  which  one  retains  the 
position  of  the  parent  cell  (which  it  replaces  as  a  new  spermatogone  destined  for  a 
succeeding  division),  while  the  other  passes  inwards,  enlarges  and  becomes  a  mother 
cell  or  primary  spertnatocyte.  This  element,  conspicuous  by  reason  of  its  size  and 
large  nucleus,  undergoes  mitotic  division  and  gives  rise  to  two  daughter  cells,  the 
secondary  spermatocytes  ox  prespennatids.  The  latter  almost  immediately  divide  and 
produce  smaller  cells,  the  spermatids,  by  whose  transformation  the  spermatozoa 
directly  arise.  It  is  important  to  note  that  the  spermatids  contain  only  one-half  of  the 
number  (probably  twenty-four)  of  chromosomes  normal  for  the  ordinary  (somatic) 
cells  of  the  human  body,  a  like  reduction  (page  9)  occurring  during  the  maturation 
of  the  ovum.  The  spermatids,  at  first  small  cells  with  round  nuclei,  elongate;  their 
nuclei  coincidently  become  oval  and  smaller,  but  rich  in  chromatin,  and  shift  to  the  end 


.("to  J  7 


Fig.  282. — Diagram  illustrating  phases  of  one  complete  cycle  of  spermatogenesis.  Sequence  of  figures 
shows  in  detail  growth  (i-6)  and  division  (7-8)  of  spermatogone;  growth  and  division  of  primary  sper- 
matocyte (9-19)  into  secondary  spermatocytes;  division  of  latter  (20-21)  into  spermatids  (22-24);  fusion 
of  these  with  Sertoli  cell  to  form  spermatoblast  (25-26);  differentiation  (27-31)  and  final  liberation  (32)  of 
spermatozoa.     {After  Ebner.) 


of  the  cell  most  removed  from  the  lumen  of  the  tubule.  The  modified  spermatids  now- 
become  closely  related  to  a  Sertoli  cell,  with  whose  cytoplasm  they  fuse.  The  struc- 
ture thus  formed,  known  as  the  spermatoblast,  consists  of  an  irregular  nucleated 
conical  mass  of  protoplasm,  with  the  inner  end  of  which  the  radiating  clusters  of  par- 
tially fused  spermatids  are  blended  (Fig.  282).  The  succeeding  changes  include  the 
transformation  of  the  elongated  nucleus  of  the  spermatid  into  tlie  head,  and  of  its 
double  centrosome  (diplosome)  into  the  neck-granules  and  (according  to  some)  the 
axial  filament  of  the  spermatozoon,  while  from  the  cytoplasm  of  the  spermatid  its 
remaining  parts  are  derived.  As  the  spermatozoa  become  more  and  more  differ- 
entiated, they  appear  as  fan-shaped  groups  in  which  the  heads  are  always  buried 
within  the  spermatoblast  and  the  tails  directed  towards  the  lumen  of  the  canal. 
After  subsequent  separation,  the  liberated  spermatic  filaments  occupy  the  centre  of 
the  tubule  as  masses  which  often  occlude  the  lumen  and  in  which  the  spermatozoa 
are  disposed  in  peculiar  whorl-like  groups.     Their  completed  development,  however, 


232 


NORMAL    HISTOLOGY. 


Fig.  283. — Human  spermatozoa  ;  one  head 
is  seen  in  profile.     X  560. 


Head  <^ 


is  deferred  until  they  reach  the  canal  of  the  epididymis,  during  the  passage  of  which 
long  and  highly  tortuous  path  they  attain  maturity  and  lose  the  cytoplasmic  remains 

of  the  spermatids  that  adhere  for  a  time  to  the 

middle-piece.    Production  of  spermatozoa,  which 

!  occurs  only  within  the  convoluted   seminiferous 

/  "      /         tubules,  does  not  involve  uniformly  all  parts  of  the 

/  /  tubule,  but  proceeds  with  wave-like  periodicity; 

f  /  consequently,  cross-sections  of  the  same  tubule 

'       '       /  taken  a  few  millimeters   apart  exhibit  different 

--^'  /  stages  of  the  spermatogenetic  cycle. 

0  ~y^~- 

Q      0      .  ^  The  spermatozoa  or  spermatic  fila- 

^  ments,    the    essential    male    reproductive 

elements,  are,  like  the  ova,  direct  derivations 

of  the  descendants  of  the  primary  indifferent 

germ-cells.    Unlike  the  ova,  how^ever,  which 

are  relatively  large  and  often  absolutely  huge 

cells,  and,  apart  from  size  and  minor  distinctions, 

fairly  similar  in  all  vertebrates,  the  spermatozoa 

present  great  diversity  in  size,  form  and  detail  and 

exhibit  a  high  degree  of  specialization.  As  ordi- 
narily seen  under  moderately  high  magnification 

(Fig.  283),  three  parts  may  be  recognized — the 

head,  the  neck  and  the  tail.      The  head  is  ovoid, 

flattened  in  front,  so  that  when  viewed  in  profile  it 

appears  pyriform.      Although  rich  in  chromatin, 

the  latter  is  not  arranged  as  threads  or  networks 

but  is  distributed  uniformly,  so  that  the  head  ap- 
pears homogeneous.     Of  the  50-60  jj.  representing 

the  approximate  entire  length  of  the  human  sper- 
matozoon, the  head  contributes  about  one-tenth 

(5-6  !J-).     The  neck,  uniting  the  head  and  the  tail, 

is  in  man  slightly  constricted  and,  therefore,  not 

easily  seen,  its  position  being  indicated  by  the  ready 

separation  of  the  head  from  the  tail  at  this  point. 

It  contains  the  minute  anterior  and  posterior  centro- 

somes  or  neck-granules.      The  tail  is  regarded  as 

composed  of  three  segments :  the  connecting  piece 

(  6  <}.) ,  the  chief  piece  (40  /-'.)  and  the  end  piece  ( 10  />-) . 

The  tail  is  traversed  throughout  its  length  by  an 

extremely  delicate  axial  fibril,  which,  with  the  ex- 
ception of  in  the  end  piece,  where  the  axial  fibril  is 

naked,  is  invested  by  an  attenuated  protoplasmic 

sheath.      In  the  connecting  piece  (middle  piece) 

the  axial  fibre  is  supposed  to  be  surrounded  by  a 

spiral  fibre,  the  posterior  limit  corresponding  with 

a  minute  end-disk.      Beyond  the  recognition  of  the 

chief-  parts  of  the  spermatozoon — the  head,  neck 

and  tail,  little  can  be  seen  of  the  above  noted  details 

unless  the  observer    be  provided  with    specially 

stained  preparations  and  lenses  of  the  highest  power 

and  most  perfect  definition.  The  living  sperma- 
tozoa, as  seen  in  fresh  semen,  display  active  move- 
ments, rapidly  changing  their  position  in  consequence  of  the  vibrations  of  the 


End  piece 
of  tail 


Fig.  2S4. — Diagram  illustrating 
structural  details  of  human  sper- 
matozoon.    {Meves.) 


THE    EPIDIDYMIS.  233 

long  motile  flagellum,  the  tail.  The  actual  rate  of  their  unobstructed  progress 
has  been  estimated  at  from  1.5-3.5  "''"''•  P^i"  minute.  Although  less  vigorous 
than  in  the  semen,  the  spermatozoa  often  display  motion  in  the  secretion  of 
the  testicle,  as  taken  from  the  epididymis.  Spermatozoa  may  continue  to 
exhibit  motion  for  a  long  time — in  the  body  for  several  days  after  death; 
in  properly  guarded  microsco[)ical  preparations  of  fresh  semen  movements 
have  been  observed  after  the  lapse  of  over  eight  days.  They  may  remain 
active  probably  for  even  a  longer  period  within  the  female  generative  tract. 
Although  resisting  a  wide  range  of  temperature,  spermatozoa  immediately 
succumb  to  aqueous  solutions  containing  acids  or  metallic  salts;  alkaline 
solutions,  on  the  other  hand,  stimulate  their  motion.  Ejaculated  semen  is 
a  composite  fluid,  consisting  of  the  secretions  of  the  testicle  diluted  with  those 
from  the  seminal  vesicles,  the  prostate  and  the  bulbo-urethral  glands.  It  has 
been  estimated  that  each  cubic  millimeter  of  semen  contains  approximately 
60,000  spermatozoa. 

THE    EPIDIDYMIS. 

The  epididymis,  the  greatly  convoluted  beginning  of  the  spermatic  duct, 
is  a  crescentic  body  that  covers  the  posterior  border  and  part  of  the  outer 
surface  of  the  testis.  Its  enlarged  upper  end  or  head,"  the  globus  major,  is 
succeeded  by  the  tapering  body,  at  the  lower  end  of  which  is  a  second  but 
smaller  enlargement,  the  globus  minor.  The  bulk  of  the  globus  major 
depends  upon  the  aggregation  of  from  twelve  to  fifteen  conical  masses,  the 
lobuli  epididymidis,  formed  by  the  efferent  ducts  and  their  tortuosities,  the 
coni  vasculosi,  that  pass  from  the  upper  end  of  the  testis  and  connect  the  rete 
testis  with  the  canal  of  the  epididymis.  The  latter,  also  called  the  ductus 
cpididymidis,  begins  in  the  globus  major,  receives  the  efferent  ducts  and  be- 
comes greatly  convoluted,  the  remarkably  wound  single  tube  measuring, 
when  unravelled,  from  5-5.5  meters  or  from  18-20  feet. 

The  efferent  ducts  form  the  conical  lobules  of  the  globus  major,  which 
masses,  together  with  the  convolutions  of  the  canal  of  the  epididymis,  are 
enclosed  by  a  fibrous  envelope  resembling  but  less  robust  than  the  capsule  of 
the  testis.  The  individual  tubules  and  convolutions  are  held  together  by  deli- 
cate vascular  connective  tissue.  The  transition  of  the  irregular  channels  of  the 
rete  testis  into  the  efferent  ducts  (.2-5  mm.  in  diameter)  is  marked  by  an 
abrupt  change  in  the  character  of  the  lining  epithelium,  the  low  cuboidal  cells  of 
the  former  giving  place  to  irregularly  ciliated  columnar  ones  within  the  ef- 
ferent ducts.  This  epithelium,  moreover,  is  composed  of  cells  of  unequal 
height,  some  forming  groups  of  tall  cylindrical  elements,  with  or  without  cilia 
and  variably  pigmented,  while  others  occur  as  groups  of  low  cuboidal  cells. 
In  consequence  of  this  inequality,  the  lumen  of  the  efferent  ducts  is  irreg- 
ular and  the  surface  of  the  mucous  membrane  modelled  with  minute  depres- 
sions corresponding  to  the  areas  covered  by  the  lower  cells.  In  some  cells 
the  border  of  cilia  is  replaced  by  clear  caps,  which  have  been  interpreted  as 
secretion.  Outside  a  well  defined  basement  membrane  the  tubules  are  sur- 
rounded with  a  layer  of  circularly  disposed  unstriped  muscle,  intermingled 
with  numerous  elastic  fibres. 

The  canal  of  the  epididymis,  from  .4-.  5  mm.  in  diameter,  is  lined 
throughout  with  stratified  columnar  epithelium,  consisting  of  a  deep  layer 
of  small  rounded  cells,  next  the  well  defined  basement  membrane,  and  a 
superficial  layer  of  tall  columnar  elements,  that  contain  pigment  particles  and 
secretion  granules.  The  free  surfaces  of  the  columnar  cells  bear  exceptionally 
long  cilia,  which,  however,  are  not  motile  and  adhere  into  conical  tufts  sur- 


234 


NORMAL    HISTOLOGY. 


Fibrous  envelope  . 


^, 


Blood- 
vessels 


Attachment  - 
to  testis 


Intertubular 

stroma 


Ductulus. 
aberrans 


/ 


mounting  the  cells.  In  places  the  epithelium  contains  minute  tubular  diver- 
ticula that  are  regarded  as  abortive  glands.  Outside  the  membrana  propria 
the  duct  is  enclosed  by  a  robust  circular  layer  of  unstriped  muscle  (15-30  /j. 
thick),  which  attains  its  greatest  development  within  the  globus  minor,  near 

the  beginning  of  the  vas 
deferens.  The  convolutions 
of  the  canal  are  held  to- 
gether and  in  place  by  inter- 
vening fibro-elr.stic  tissue. 

The  blood-vessels 
supplying  the  testicle  are 
branches  from  the  spermat- 
ic and  deferential  arteries, 
those  from  the  former  being 
distributed  especially  to  the 
testis  and  those  from  the 
latter  to  the  epididymis. 
The  spermatic  branches 
enter  the  mediastinum  and 
break  up  into  superficial 
and  deep  twigs  that  follow 
the  tunica  albuginea  and  the 
septa  respectively.  They 
continue  within  the  tracts 
of  intertubular  connective 
tissue  and  ultimately  form 
rich  capillary  networks  en- 
closing the  seminiferous 
tubules,  immediately  out- 
side the  basement  mem- 
brane. The  arteries  dis- 
tributed to  the  epididymis 
course  within  the  intertubu- 
lar stroma  and  likewise  re- 
solve into  capillaries  that 
enclose  the  efferent  ducts  and  the  convolutions  of  the  canal  of  the  epididy- 
mis. The  veins  arise  from  the  capillary  networks  ;  those  from  the  testis, 
superficial  and  deep,  emerge  at  the  mediastinum  and,  joining  with  those  from 
the  globus  major,  concentrate  into  several  stems  of  considerable  size  that 
ascend  within  the  spermatic  cord  in  the  anterior  part  of  the  pampiniform 
plexus.  The  veins  from  the  body  and  tail  of  the  epididymis  unite  into  a 
smaller  group  that  ascend  in  the  posterior  part  of  the  plexus. 

The  lymphatics  of  the  testis  begin  in  the  connective  tissue  surround- 
ing the  tubules  and  follow,  in  a  general  way,  the  course  of  the  veins  as  a 
superficial  and  a  deep  set.  They  emerge  from  the  mediastinum  as  six  to 
eight  relatively  large  trunks,  to  which  the  lymphatics  of  the  epididymis  are 
tributar}'-,  and  accompany  the  veins  in  the  cord. 

The  nerves  of  the  testicle  are  chiefly  sympathetic  fibres  destined  for 
the  walls  of  the  blood-vessels  and  the  unstriped  muscular  tissue  of  the  epi- 
didymis. They  form  plexuses  enclosing  microscopic  ganglia  around  the  larger 
\essels.  The  relation  between  the  terminal  fibres  and  the  tubules  includes 
epilemmar  fibrils  on  the  exterior  of  the  basement  membrane  and,  perhaps, 
a  few  hypolemmar  ones  that  penetrate  between  the  epithelial  cells. 


^S^^       Vas  deferens 
Fig.  285. — Section  across  lower  part  of  epididymis.     X  15. 


THE   SPERMATIC    DUCTS. 


THE   APPENDAGES    OF   THE   TESTICLE. 

Under  this  heading  are  included  several  vestigial  organs  that  remain  for 
a  variable  period,  some  throughout  life,  as  more  or  less  conspicuous  bodies 
attached  to  the  testis  or  the  epididymis.  They  claim  attention  not  only  on 
account  of  their  morphological  relations,  but  also  because  they  may  become 
the  seat  of  pathological  changes.  The  most  important  are  :  the  appe?idix 
testis,  the  appendix  epididyinidis,  \\\&  paradidymis  and  the  ducttdi  aberrantes. 

The  appendix  testis,  also  called  the  icnsta/ked  or  sessile  hydatid,  is  a 
small  but  fairly  constant  body,  5-10  mm.  in  length  and  less  than  half  as  broad, 
fixed  to  the  upper  pole  of  the  testis.  It  consists  of  a  vascular  connective  tissue 
stroma  in  which  lies  a  minute  canal  of  variable  size  and  extent,  lined  with 
columnar  epithelium.  The  appendage  represents  the  atrophic  upper  end  of 
the  Mijllerian  duct,  one  of  a  pair  of  foetal  tubes  that  in  the  female  embryo 
give  rise  to  the  oviducts,  the  uterus  and  the  vagina. 

The  appendix  epididymidis,  or  sta/ked  hydatid,  is  a  small  pyriform 
sac,  from  3—4  mm.  in  length,  containing  a  clear  fluid  and  lined  with  cuboidal 
epithelium.  It  is  variable  in  form,  size  and  number,  two  or  more  sometimes 
being  present,  and  is  probably  derived  from  the  tubules  of  the  foetal  Wolffian 
body. 

The  paradidymis,  or  orga)i  of  Giraldes,  consists  of  an  irregular  group 
of  blind  tubules,  from  5-6  mm.  in  length,  that  lies  within  the  lower  end  of 
the  spermatic  cord,  abo\'e  but  close  to  the  head  of  the  epididymis.  The 
tubules  are  lined  with  cuboidal  or  columnar  ciliated  epithelium  and  are 
derivatives  of  the  Wolffian  tubules. 

The  ductuli  aberrantes  include  tubular  appendages,  usually  an  upper 
and  a  lower,  that  extend  for  an  uncertain  distance  within  the  epididymis 
among  the  con\^olutions  of  its  duct.  The  tubules  are  lined  with  ciliated 
columnar  or  cuboidal  epithelial  cells  and  are  regarded  as  originating  from 
the  atrophic  tubules  of  the  Wolffian  body. 

THE  SPERMATIC  DUCTS. 

The  spermatic  duct,  in  the  more  usual  and  restricted  sense,  is  one  of  a 
pair  of  tortuous  canals  that  connect  the  epididymis  with  the  urethra  and  thus 
provide  channels  for  the  escape  of  the  secretion  of  the  sexual  glands.  Each 
duct  is  conventionally  described  as  composed  of  the  vas  deferens  and  its 
ampulla  and  the  ejaciilatory  duct ;  at  the  upper  end  of  the  latter  the  sper- 
matic duct  is  connected  with  the  seminal  vesicle,  a  saccular  organ  derived  as 
an  outgrowth  from  the  main  canal. 

The  Vas  Deferens. — This  tube  extends  from  the  epididymis  to  the 
ejaculatory  duct  and,  when  straightened  out,  measures  some  45  cm.  (18  in.), 
thus  contributing  almost  the  entire  length  of  the  spermatic  duct.  Its 
diameter  is  from  2-3  mm.  Within  the  spermatic  cord  (pars  funicularis)  the 
vas  occupies  a  position  behind  the  other  constituents  of  the  cord  and  may 
be  recognized  by  the  hard  cord-like  feel  imparted  by  its  thick  fibro-muscular 
wall.  The  latter  (1-1.5  mm.  thick)  encloses  a  narrow  lumen  and  consists 
of  three  coats — the  mucous,  muscular  and  fibrous.  The  mucous  coat  is 
clothed  with  epithelium  which  for  a  considerable  distance  resembles  that  of 
the  canal  of  the  epididymis,  being  made  up  of  a  superficial  layer  of  columnar 
and  a  deep  one  of  small  rounded  cells.  Throughout  the  upper  part  of  the 
duct,   however,   the  cells  are  lower,   without  cilia,   and   approach  a  simple 


236 


NORMAL  HISTOLOGY. 


cuboidal  type.  Numerous  particles  of  pigment  are  common  in  their  cyto- 
plasm. The  tunica  propria  contains  a  dense  network  of  elastic  fibres  within 
its  outer  zone.  The  unusually  robust  niitsciilahire  of  the  vas,  from  .8-1.2 
m.m.  thick,  constitutes  approximately  four-fifths  of  the  entire  wall  and  includes 

unstriped  fibres  arranged   as 

an  outer  longitudinal,  a  mid- 

-„.,  .V"y  ^  ^".;-  die    circular,    and    an    inner 

longitudinal  layer,  the  last 
mentioned  layer  being  much 
less  developed  than  the  outer 

Fibrous  coal ..  and  middle  Strata.   Thejibrous 

coat  is   composed   of    closely 

Lumen  — arranged    bundles   of   fibrous 

tissue  and  many  elastic  fibres. 
In  the  funicular  part  of  the 
duct,  between  the  epididymis 
and  the  abdominal  wall,  the 
fibrous  coat  contains  strands 
of  unstriped  muscle,  which 
belong  to  the  coverings  of  the 
cord  and  constitute  the  so- 
called  internal  cremaster. 

The  ampulla,  the  some- 
what flattened  fusiform  en- 
largement of  the  vas  just 
before  it  becomes  the  ejaculatory  duct,  is  uneven  and  humpy  in  contour 
owing  to  the  sacculations  and  tortuosity  of  the  canal  and  the  short  diver- 


Outer 
longitudinal   — 
muscle 

Circular 

muscle 

Inner 

longitudinal  ^^ 

muscle 

Mucous  ^ 

membrane 


Fig.  286. — Cross-section  of  vas  deferens.     X  15. 


— Net-work  of 
i\      cut  ridges  - 

—Mucous  coat 


Main  lumen 


Lateral 


C  -  V                   ^   ^i-Gtr^  "">  ^m^^^^V-Jj                 J'^    'si'L-^i  'i   >  diverticulum 

<■     \  "   — =;ii- '-^ Circular  muscle 

>,  ^  '-'          I- 

\>  '        ' 

■',i'\.  ■^^- Longitudinal 

"■»--"  "^       .                 niubcle 


Fig.  287. — Transverse  section  of  ampulla  of  spermaiic  duct  (vas  deferens).     X  18. 

ticula  that  pass  from  the  main  duct  at  various  angles.  In  its  general  struc- 
ture the  ampulla  corresponds  with  the  vas  deferens,  its  walls,  however, 
possessing  a  much  thinner  and  less  regular  muscular  coat — the  inner  longi- 
tudinal layer  disappearing,  the  outer  one  being  imperfect,  and  the  orderly 


THE   SPERMATIC    DUCTS. 


237 


disposition  of  the  circular  fil^res  being  disturbed  by  oblique  bundles.  The 
mucous  membrane  is  modelled  by  numerous  ridges  and  depressions  and 
covered  with  a  single  layer  of  low  columnar  nonciliated  epithelial  cells. 

The  Ejaculatory  Duct. — This,  the  terminal  segment  of  the  spermatic 
canal,  although  apparently  formed  by  the  union  of  the  duct  of  the  seminal 
vesicle  and  the  vas  deferens,  is  the  morphological  continuation  of  the  vas, 
from  which,  when  still  represented  by  the  embryonic  Wolffian  duct,  the 
seminal  vesicle  develops  as  an  outgrowth.  The  ejaculatory  duct  penetrates 
part  of  the  prostate  gland  and  ends  in  the  urethra  by  a  minute  opening  sit- 
uated on  the  urethral  crest,  at  the  side  of  the  orifice  of  the  prostatic  utricle. 
It  possesses  a  structure  essentially  the  same  as  that  of  other  parts  of  the  sper- 
matic canal,  its  walls,  however,  being  thinner  than  in  the  ampulla  in  conse- 
quence of  the  diminished  thickness  and  incompleteness  of  the  muscle.  On 
reaching  the  ejaculatory  duct  the  longitudinal  muscle  disappears  and  even 
the  remaining  circular  bundles  become  greatly  reduced  and  intermingled 
with  fibrous  tissue  which  almost  replaces  them.  The  surface  of  the  duct, 
particularly  along  its  upper  wall,  is  broken  by  minute  depressions  and  diver- 
ticula which  recall,  in  miniature,  those  modelling  the  seminal  vesicles.  Some 
of  these  are  branched  tubules  and  recall  tubo-alveolar  glands.  The  char- 
acter of  the  epithelium  is  inconstant,  in 
places  the  lining  being  a  single  and  in 
others  a  double  layer  of  low  columnar 
cells;  within  a  short  distance  of  the 
end  of  the  duct,  the  epithelium  assumes 
the  transitional  type  found  in  the 
prostatic  urethra. 

The  Seminal  Vesicle. — This 
organ,  one  df  a  pair  of  sacculated 
appendages  of  the  spermatic  ducts, 
lies  behind  the  bladder  and  in  front 
of  the  rectum.  Its  general  form  is 
pear-shaped,  with  the  base  directed  up- 
wards and  outwards  and  the  abruptly 
tapering  lower  end  converging  to  join 
the  spermatic  duct.  When  divested 
of  the  fibro-muscular  capsule  that 
blends   the  divisions  into  a   knobbed 

mncc       tVio     /-,1-mn     m tvt-     Kfi     c<=>nQ rafprl      FiG.  2S8. — Cross-seclioii  of  semitial  vesicle,  show- 

mass,    tne    oigan    may    oe    sepaiatea  ing  modelling  of  mucous  surface,    x  12. 

into  a  chief  duct  and  dwerhcitla,  all  of 

which,  after  repeated  windings,  end  blindly.  The  lumen  of  the  chief  duct, 
as  seen  in  section  (Fig.  289),  is  irregular,  constrictions  and  dilatations 
following  one  another  with  little  order.  The  seminal  vesicle  contains  a  light 
brownish  fluid  in  which  spermatozoa  are  usually  found  during  the  period 
of  se.xual  -activity. 

In  its  general  structure,  the  seminal  vesicle  resembles  closely  the  ampulla 
of  the  vas  deferens,  possessing  a  robust  muscular  coat  composed  of  an  inner 
circular  and  an  outer  longitudinal  layer  of  unstriped  tissue.  The  mucous 
membrane  is  conspicuously  modelled  by  numerous  ridges  and  pits,  so  that 
in  sections  it  appears  honeycombed  (Fig.  288).  The  surface  of  the  larger 
ridges  is  covered  by  two  or  three  layers,  that  of  the  pits  and  diverticula  by  a 
single  layer,  of  low  columnar  epithelial  cells,  many  of  which  contain  secre- 
tion-particles. Although  definite  glands  are  wanting,  minute  branched 
tubular  canals,  lined  with  low  columnar  epithelium  containing  goblet-cells 


Pitted     / 

surface  of/_^ 

mucous 
membrane 


238 


NORMAL   HISTOLOGY. 


and  other  evidences  of  secretory  activity,  extend  into  the  mucosa  from  the 
bottom  of  the  deeper  recesses.  Pigment  granules  are  also  constant  after 
the  advent  of  sexual  maturity.  The  tunica  propria  is  rich  in  elastic  fibres. 
The  fluid  produced  within  the  seminal  vesicles  is  of  importance  probably  not 
only  in  diluting  the  secretion  of  the  testicle  and  supplying  a  medium  favor- 
able for  the  motility  of  the  spermatozoa,  but  also  in  completing  the  volume  of 
fluid  favorable  for  ejaculation  (Waldeyer).  The  spermatic  ducts,  and  not 
the  vesicles,  serve  as  the  chief  reservoirs  for  the  spermatozoa. 

The  blood-vessels  supplying  the  spermatic  duct  and  the  seminal 
vesicle  give  off  twigs  that  enter  the  walls  and  break  up  into  capillary  net- 
works within  the  muscular  and  mucous  coats.  That  within  the  latter  occu- 
pies the  superficial  part  of  the  tunica  propria,  immediately  beneath  the 
basement  membrane.  The  veins  begin  within  the  deeper  part  of  the 
mucosa  and,  after  piercing  the  walls  of  the  duct  and  vesicle,   unite  into  a 


\i    '  ''^f    ^^,.^Partition  separating  adjacent  diverticula 
its  on  mucous  surface 


Epithelium 


Mucous  coat 


Circular 

muscte 
Longitudinal 

muscle 


"Fibrous  coat 


■&^ 


Fig.  289.  —Wall  of  seminal  vesicle  in  longitudinal  section,  showing  pitting  of  mucous  coat.    X  45- 


superficial  network,  following  the  vas  as  the  deferential  plexus  and  surround- 
ing the  vesicle  as  the  seminal  plexus.  Within  the  spermatic  cord  the  former 
communicates  with  the  pampiniform  plexus,  the  component  veins  of  which 
are  distinguished  by  unusually  well  marked  muscle. 

The  lymphatics  of  the  seminal  duct  and  vesicle  are  numerous  and 
arranged  as  a  deeper  and  a  superficial  set.  The  former  arise  from  lymph- 
channels  within  the  mucous  and  muscular  coats  and  join  the  superficial  net- 
work, outside  the  dense  walls,  from  which  efferent  trunks  pass  to  the 
lymph-nodes. 

The  nerves  of  the  duct  and  vesicle  are  derived  chiefly  from  the 
hypogastric  sympathetic  plexus;  they  surround  the  blood-vessels  with 
plexiform  meshes  from  which  fibres  pass  into  the  muscular  tissue  where 
they  form  the  dense  plexus  inyospermaticus.  The  latter  sends  fibres  to 
supply  the  unstriped  nmscle,  while  others  penetrate  the  mucous  membrane 
to  end  mostly  within  the  tunica  propria,  some  fibrils  gaining,  perhaps,  an 
intraepithelial  position. 


THE   MALE   COPULATIVE   ORGAN.  239 


THE    PENLS. 

The  male  copulative  oroan  cf)nsists  of  three  cylinders  of  erectile  tissue — 
the  paired  corpora  cavernosa  and  siniJ^le  corpus  spongiosum — united  with  one 
another  and  invested  by  coverings  of  fascia  and  skin.  The  anterior  or  upper 
and  flattened  surface  of  the  penis  is  formed  by  the  corpora  cavernosa;  the  pos- 
terior or  under  surface  corresponds  to  the  corpus  spongiosum,  which  is  trav- 
ersed by  the  urethra.  The  q.o\\\q-2X  glans  penis,  forming  the  free  end  of  the 
organ,  is  continuous  with  the  spongy  body  which  it  resembles  in  structure. 

Each  of  the  cylinders  of  erectile  tissue  is  enclosed  by  a  robust  sheath, 
the  tunica  albuginea,  composed  of  dense  fibrous  tissue,  intermingled  with 
fine   elastic   fibres  but   no   muscle.      The   sheath   surrounding   the   corpora 


Subcutaneous  tissue^:^"'^''"-'^^^^^*-"^*^'^'*'  '  "■-~'^^^^^kj^Deep  dorsal  vein 


Skin 

'^-Tunica  albugiiiea 
Septum  -if- 


Corpus  cavernosmn 


Superficial  fascia 


Urethra    ''■'<"_ 

Corpus  spongiosum 


Fig.  290. — Transverse  section  of  penis  of  child.     X  lo. 

cavernosa,  which  includes  an  outer  longitudinal  and  an  inner  circular  laver 
and  in  places  attains  a  thickness  of  over  i  mm. ,  is  much  stronger  than  that 
enclosing  the  spongy  body;  it  is,  however,  imperfect  along  the  opposed 
median  surfaces  of  the  two  cylinders,  where  it  forms  the  pectiniforni  septum. 
From  the  inner  surface  of  the  tunica  albuginea  fibrous  septa  and  trabecular 
are  given  off  which  constitute  the  framework  supporting  the  vessels  and  nerves 
and  enclosing  the  characteristic  blood-spaces  of  the  erectile  tissue.  Numerous 
bundles  of  unstriped  muscle,  irregularly  disposed,  occupy  the  fibrous  trabeculse 
and  plates  that  separate  the  venous  spaces,  which  are  thus  surrounded  by 
imperfect  layers  of  contractile  tissue.  The  trabecular  muscle  is  most  abundant 
within  the  cavernous  and  spongy  bodies,  and  least  so  within  the  glans. 

The  arteries  conveying  blood  to  the  erectile  tissue  are  of  two  kinds: 
those  coursing  within  the  trabeculae  and  nourishing  the  tissues,  vasa  mctritia, 
and  those  carrying  blood  primarily  to  the  venous  lacume.     The  latter  are 


240 


NORMAL    HISTOLOGY. 


connected  with  the  arteries  either  directly,  by  minute  channels,  or  by  inter- 
vening capillaries.  Within  the  trabeculae  of  the  deeper  parts  of  the  erectile 
masses,  short  tortuous  branches,  arterice  helicincB,  are  given  off ;  in  the 
relaxed  condition  these  are  twisted  and  project  into  the  blood-spaces.  Both 
the  arterise  nutritise  and  helicinae  finally  directly  communicate  by  minute 
canals  with  the  deeper  lacunae  of  the  cavernous  tissue.  The  arteries  of  the 
erectile  tissue  are   remarkable  for  the  unusual   thickness  of  their  circular 


Central  blood-spaces       Inner  peripheral  spaces       Outer  peripheral  spaces 


Trabeculas  Bundles  of  muscle  Dense  fibrous  tissue  of 

tunica  albuginea 

Fig.  2gi. — Transverse  section  through  periphery  of  corpus  cavernosum,  showing  erectile  tissue.    X  5o. 

muscle.  In  places  the  intima  likewise  exhibits  excessive  thickness  due  to 
accumulation  of  longitudinal  muscle,  such  local  augmentations  producing 
cushion-like  bulgings  that  encroach  upon  the  lumina  of  the  arteries. 

The  lacunae,  the  blood-spaces  that  occupy  the  interstices  between  the 
trabeculae,  are  regarded  as  venous  networks  which  communicate  with  the 
arteries,  on  the  one  hand,  and  with  the  radicles  forming  the  veins  on  the 
other.  Beyond  the  single  layer  of  lining  endothelial  plates  they  possess  no 
special  wall.  Their  form  and  size  depend  upon  the  degree  of  distention, 
when  containing  little  blood  being  often  mere  slits  or  irregular  stellate  clefts, 
while  when  filled  they  become  more  cylindrical.  Three  tracts  may  be  dis- 
tinguished: {a)  a  narrow  outer  peripheral  zone  oi  almost  capillary  spaces, 
for  the  most  part  narrow  and  triangular  in  outline;  (^b)  an  inner  peripheral 
zone  of  larger  spaces  of  uncertain  form  and  from  .15-. 20  mm.  in  diameter; 
and  (c)  a  central  zone  of  still  more  extensive  spaces  ( 1-3  mm. )  enclosed  by 
relatively  thin  lamellae  and  trabeculae.  The  deep  veins  draining  the  erectile 
cylinders  do  not  directly  open  from  the  blood-spaces,  but  are  formed  by 
tributaries  of  various  size  that  begin  as  apertures  in  the  walls  of  the  lacunae 
of  which  they  are  extensions.  The  tributaries  of  the  superficial  venous 
trunks,  as  the  dorsal  veins,  arise  chiefly  from  the  venous  networks  of  the 
peripheral   zone.      The  veins  possess  unusually  strong  muscular  coats  and 


THE   PROSTATE   GLAND.  241 

exhibit  local  cushion-like  thickenings  of  the  intima,  similar  to  but  less  marked 
than  those  seen  in  the  arteries.  The  erectile  tissue  of  the  corpus  spongiosum 
includes  that  of  the  urethral  mucosa,  produced  by  the  unusual  abundance  of 
the  venous  channels,  and  that  of  the  spongy  body  proper,  a  surrounding  tract 
having  much  the  character  of  the  corpora  cavernosa.  The  spongy  body  is 
distinguished  by  the  stoutness  of  its  trabecuke  and  the  small  size  of  its  venous 
spaces;  further,  by  the  absence  of  arteries  opening  directly  into  the  lacunae. 

The  lymphatics  of  the  penis  are  disposed  as  superficial  and  deep 
vessels.  The  latter  are  particularly  numerous  in  the  periphery  of  the  glans 
and  send  tributaries  to  aid  in  forming  a  deep  dorsal  lymph-stem  along  the 
corresponding  vein.  The  superficial  lymphatics  are  directed  chiefly  to  a 
superficial  dorsal  trunk  that  accompanies  the  superficial  vein  and  begins  by 
the  confluence  of  networks  within  the  integument. 

The  nerves  of  the  penis  include  both  spinal  and  sympathetic  fibres, 
the  former  from  the  ilio-inguinal  and  the  pudic  nerves  and  the  latter  from 
the  hypogastric  plexus.  The  integument  of  the  body  and  glans  and  prepuce 
is  supplied  by  the  dorsal  nerv^es.  The  cylinders  of  cavernous  tissue  also 
receive  twigs  from  the  pudic  nerves,  the  bulbar  branches  of  which  pass  to 
the  bulbus  urethrae  and  in  addition  supply  the  mucous  membrane  of  the 
urethra.  Each  corpus  cavernosum  receives  a  deep  branch  from  the  dorsal 
nerve.  The  sympathetic  fibres,  destined  for  the  blood-vessels  and  unstriped 
muscle  of  the  erectile  tissue,  continue  from  the  hypogastric  to  the  cavernous 
plexus;  here,  joining  the  dorsal  nerves  of  the  penis,  twigs  are  sent  to  the 
corpora  cavernosa,  some  terminating  in  the  spongy  body.  Close  networks 
of  nonmedullated  fibres  have  been  followed  into  the  involuntary  muscle 
within  the  blood-vessels  and  the  trabeculse  of  the  erectile  tissue.  Certain 
cerebro-spinal  fibres,  known  as  the  nervi  erigentes,  from  the  third  and  fourth 
sacral  nerves,  are  supposed  to  be  especially  concerned  in  erection ;  they  are 
conveyed,  in  company  with  the  sympathetic  fibres,  along  the  paths  of  the 
cavernous  plexus.  In  addition  to  the  more  usual  terminations,  the  skin  of 
the  glans  and  prepuce  is  provided  with  special  nerve-endings — tactile  corpus- 
cles and  genital  corpuscles — Iving  within  the  papillae  and  the  Pacinian  bodies 
within  the  subcutaneous  stratum.  The  path  of  the  sensory  impulses  lies 
within  the  dorsal  nerves  of  the  penis. 

THE    PROSTATE   GLAND. 

Although  developed  as  an  appendage  of  the  urinary  tract  and  not  directly 
as  part  of  the  sexual  apparatus,  the  prostate  is  functionally  so  closely  related 
to  the  generative  organs,  that  it  may  appropriately  be  regarded  as  one  of  the 
accessory  glands,  the  others  being  the  bulbo-urethral  glands. 

The  prostate  gland  resembles  in  form  an  inverted  Spanish  chestnut,  the 
base  being  attached  to  the  under  surface  of  the  bladder  and  the  small  end,  or 
apex,  directed  downwards.  It  is  tra\'ersed  from  base  to  apex  by  the  urethra, 
and  from  behind  to  the  urethra  by  the  ejaculatory  ducts.  The  prostate  is  a 
tubo-alveolar  gland  and  made  up  of  three  chief  components — the  connective 
tissue  framework,  the  involuntary  muscle,  and  the  glandular  tissue.  Of  these 
the  glandular  tissue  constitutes  a  little  more  than  one  half  of  the  entire  organ 
and  the  connective  tissue  and  the  muscle  each  somewhat  less  than  one  quarter. 

The  connective   tissue   framework   includes  an    external    fibro-elastic 

envelope,  the  capsule  proper,  and  a  median  septum  which  encloses  and  blends 

with  the  walls  of  the  urethra.      Between  these  denser  lamellae,   numerous 

radiating  partitions  subdi\nde  the  organ  into  from  thirty  to  forty  pyramidal 

16 


242 


NORMAL   HISTOLOGY. 


lobules  occupied  by  glandular  tissue.  The  involuntary  muscle,  embedded 
within  the  capsule  and  the  ramifications  of  the  connective  tissue  framework, 
surrounds  the  gland-substance  as  a  superficial  layer,  from  which  a  median 
septum  (about  2  mm.  wide)  extends  ventro-dorsally  and  encloses  the  urethra 
in  an  annular  thickening.  The  interior  of  the  prostate,  therefore,  is  occu- 
pied by  a  dense  fibro-muscular  core,  or  "nucleus,"  in  which  the  glandular 
tissue  is  represented  by  the  narrow  prostatic  ducts  passing  towards  the 


Folds  of 
mucous  membrane 


Lumen  of 
urethra 


Urethral  mucous 
membrane 

Urethral  crest 
Prostatic 
utricle 


Ejaculatory  ducts  , 


Fig.  292. — Section  across  prostatic  urethra,  above  entrance  of  ejaculatory  ducts,  showing  urethral  crest 

with  prostatic  utricle.    X  lo. 

urethra.  The  muscle  is  not  limited  to  the  foregoing  positions,  but  is  found 
between  the  divisions  of  the  gland-tissue,  the  interalveolar  septa  consisting 
in  places  largely  of  the  variously  disposed  muscle-bundles. 

The  glandular  tissue  embraces  from  30-50  lobules  drained  by  a 
number  (15-30)  of  excretory  tubules,  th.Q  prostatic  ducts,  that  open  into 
the  prostatic  urethra  in  the  groove  on  either  side  of  the  median  elevation,  the 
coUiculus.  Beginning  at  their  narrow  orifices,  the  ducts  pass  outward  into 
the  lobules,  after  a  course  of  about  i  cm.  dividing  into  tubules  that  repeatedly 
branch  and  expand  into  the  terminal  alveoli.  Throughout  the  greater  part 
of  their  course  the  wavy  ducts  are  beset  with  succular  and  tubular  diverticula, 
simple  or  compound,  that  give  the  canals  irregular  lumina  and  constitute  the 
dzict  alveoli  as  distinguished  from  the  tenninal  alveoli.  The  latter  form  a 
series  of  irregularly  branched  tubular  and  saccular  spaces  hned  with  a  single 
or  imperfect  double  layer  of  columnar  epithelial  cells,  the  secreting  elements 
of  the  gland.  By  communication  the  alveoli  form  in  places  networks  of 
spaces  of  variable  form  and  size.  The  epithelium  of  the  prostatic  ducts  and 
their  diverticula  corresponds  with  that  lining  the  terminal  alveoli,  the  change 
into  the  transitional  variety  found  in  the  prostatic  urethra  not  occurring  until 
very  near  the  termination  of  the  ducts. 

Peculiar  concretions,  known  also  as  "prostatic  calculi"  or  "amyloid 
bodies,"  are  almost  constantly  present  within  some  of  the  alveoli  of  the  adult 
organ,  especially  in  advanced  life.  These  bodies  (Fig.  294),  round  or  oval  in 
outline  and  very  variable  in  size  (.2-1  mm.  and  more),  usually  exhibit  a  faint 


THE    PROSTATE   CiEAND. 


243 


concentric  striation  and  a  light  brownish  color.      Their  nature  is  uncertain, 
but  they  probably  consist  of  modified  secretion  and  contain  a  colloid  substance. 


Terminal  duct  opening  into  alveoli 


Involuntary  muscl 


Interlobular 
connective 
tissue 


Alveoli 


Blood-vessd-^^-^^-^l^jP^^v^^,^^.     _    ^ 

Fig.  293. — Transverse  section  of  prostate  gland.      <  75. 

The  secretion  of  the  prostate  gland  is  turbid  or  milky  in  appearance, 
thin  in  consistence,  slightly  alkaline  in  reaction  and  possesses  a  characteristic 


■Muscle  cell 


Fpithelium 
'lining:  alveoli 


/''^  \       — t — ,^^^SS_Interalveolar 

f  •'■^i.^.S^JUdfiH  tissue 


Blood-vessel 


Fig.  294. — Section  of  prostate  gland,  showing  details  of  alveoli.      <  270. 


odor.  It  is  discharged  into  the  urethra  and  mingled  with  the  fluid  entering 
by  the  spermatic  ducts  during  ejaculation,  and  probably  serves  an  important 
purpose  in  facilitating  and  perhaps  stimulating  the  motility  of  the  sperma- 


244  NORMAL    HISTOLOGY. 

tozoa.  The  ' '  sperm  crystals, ' '  formed  in  semen  on  standing  and  attributed 
to  the  products  of  the  prostate,  are  not  found  in  the  prostatic  secretion  during 
life,  although  frequently  present  in  the  gland  after  death. 

The  blood-vessels  supplying  the  prostate  enter  the  periphery  of  the 
gland  at  various  points,  particularly  in  company  with  the  ejaculatory  ducts. 
The  interlobular  twigs  follow  the  septa  and  eventually  break  up  into  capillary 
networks  that  surround  the  alveoli.  The  numerous  venous  radicles  form 
close  meshworks  within  the  grandular  tissue  and  around  the  ducts.  The 
larger  veins  leave  the  deeper  parts  of  the  organ  on  each  side  and  unite  into  a 
plexus  within  the  capsule,  from  which  pass  emergent  trunks. 

The  lymphatics  arise  in  lymph-channels  around  the  alveoli.  From 
the  deeper  networks  stems  pass  to  the  surface,  where  they  form  a  second 
and  superficial  network  from  which  efferents  course  in  various  directions. 

The  nerves  of  the  prostate  are  chiefiy  sympathetic  fibres  derived  from 
the  hypogastric  plexus,  numerous  microscopic  ganglia  occurring  along  their 
course.  Their  ultimate  distribution  is  largely  to  the  walls  of  the  blood- 
vessels and  to  the  unstriped  muscle,  additional  fibres  being  traceable  into 
the  glandular  tissue,  outside  the  basement  membrane  of  the  alveoli.  Sen- 
sory endings  include  special  terminations  in  the  form  of  lamellated  corpuscles, 
end-bulbs,  and  peculiar  encapsulated  endings,  which  are  modifications  of  the 
Pacinian  and  Krause  corpuscles.  These  peculiar  end-organs  are  found 
chiefly  within  the  fibrous  capsule. 

THE  BULBO-URETHRAL  GLANDS. 

The  bulbo-urethral  or  Cowper''  s  glands  are  two  small  bodies  situated  on 
the  under  surface  of  the  membranous  portion  of  the  male  urethra,  one  on 
either  side  of  and  close  to  the  mid-line.  In  general  form  and  size  (5—8  mm. 
in  diameter)  they  resemble  a  pea,  although  their  contour  is  irregular. 

The  ducts  of  the  glands,  about  1.5  mm.  in  diameter  and  from  3-4  cm. 
in  length,  run  forwards  and  medially  and  open  by  small  slit-like  orifices, 
often  by  a  common  opening,  on  the  lower  wall  of  the  bulbus  part  of  the 
spongy  urethra.  The  glands  are  mucous  tubo-alveolar  in  type,  their  ter- 
minal divisions  ending,  after  more  or  less  branching,  in  irregularly  sacculated 
compartments.  In  places  the  latter  communicate  by  means  of 'a  reticulum 
of  connecting  canals.  The  alveoli  are  lined  with  low  columnar  or  pyriform 
cells,  among  which  mucus-secreting  elements  are  plentiful.  The  cuboidal 
epithelium  that  lines  the  smaller  ducts,  as  well  as  the  dilatations  connected 
with  them,  is  succeeded  by  columnar  cells  within  the  larger  ducts  until  near 
their  termination,  where  the  simple  epithelium  is  replaced  by  two  or  more 
rows  of  cells.  The  divisions  of  the  gland  are  united  by  intertubular  con- 
nective tissue  and  invested  by  a  fibrous  envelope  containing  a  considerable 
quantity  of  unstriped  inuscle  intermingled  with  striated  fibres  derived  from 
the  surrounding  compressor  urethrae  muscle.  The  secretion  of  Cowper' s 
glands  is  clear  and  viscid  and  of  alkaline  reaction. 

The  blood-vessels  supplying  the  bulbo-urethral  glands,  branches 
from  the  arteries  of  the  bulb,  form  capillaries  that  enclose  the  alveoli  and 
diverticula.  The  veins  begin  in  the  interalveolar  tissue  and  are  tributary  to 
those  from  the  bulbus  part  of  the  spongy  body.  The  lymphatics  arise 
from  networks  of  lymph-channels  in  the  interalveolar  connective  tissue  and 
join  into  efferents  to  the  internal  iliac  lymph-nodes.  The  nerves  are  from 
the  pudic  and  include  both  meduUated  and  nonmeduUated  fibres,  the  latter 
being  principally  from  the  sympathetic. 


THE  FEMALE  REPRODUCTIVE  ORGANS. 

The  reproductive  ory-ans  of  the  female  include  two  groups_,  the  internal 
and  the  external  organs.  The  internal  organs  are:  the  sexual  glands,  the 
ovaries,  which  produce  the  ova;  the  oviducts  or  Fallopiayi  tubes,  the  canals 
conveying  the  ova  after  these  are  liberated  from  the  ovaries;  the  uterus  and 
the  vagina,  the  passage  which  embraces  the  lower  end  of  the  uterus  above 
and  ends  below  within  the  genital  cleft.  The  oviducts,  uterus  and  vagina 
represent  the  excretory  canals  of  the  sexual  glands  which  in  the  embryo,  as 
the  Miillerian  ducts,  for  a  time  are  separate.  After  fusion  of  their  lower  seg- 
ments has  taken  place,  the  unpaired  canal  thus  formed  becomes  the  vagina 
and  the  uterus,  the  latter  being  a  specialized  segment  for  the  reception  and 
retention  of  the  fertilized  ovum  during  gestation.  The  external  organs, 
termed  collectively  the  vulva  (pudendum  muliebre)  include:  the  clitoris,  the 
labia  and  the  thereby  enclosed  vestibule  and  vaginal  orifice,  and  the  glands 
of  Bartholin.  Although  morphologically  belonging  to  the  integument,  the 
mammary  glands  may  be  conveniently  regarded  as  appendages  to  the  repro- 
ductive organs. 

THE   OVARIES. 

The  ovary,  one  on  either  side  of  the  body,  is  the  sexual  gland  proper, 
within  and  from  which  are  developed  and  liberated  the  mature  maternal 
germ-cells,  the  ova.  It  is  a  solid  body,  resembling  in  form  a  large  almond, 
and  in  the  adult  lies  against  or  near  the  lateral  pelvic  wall  invested  by  modified 
peritoneum  continued  from  the  posterior  surface  of  the  broad  ligament  of  the 
uterus.  That  portion  of  the  attached  anterior  border  through  which  the 
vessels  and  nerves  enter  and  emerge  is  known  as  the  hilum.  The  surfaces  of 
the  mature  ovary  are  not  even,  as  in  early  life,  but  modelled  by  rounded  ele- 
vations of  uncertain  size  and  number  and  by  irregular  pits  and  scars.  The 
elevations  are  due  to  the  underlying  egg-follicles  in  different  stages  of  growth, 
while  the  scar-like  areas  indicate  the  position  of  corpora  lutea  which  replace  the 
ruptured  egg-follicles.  The  average  dimensions  of  the  adult  ovary  are:  36 
mm.  in  length,  18  mm.  in  breadth  and  12  mm.  in  thickness.  After  cessation  of 
menstruation,  about  the  forty-fifth  year,  the  ovary  decreases  in  size  and  weight, 
in  old  women  being  reduced  to  one  half  or  less  of  its  normal  proportions. 

The  ovary  consists  of  two  principal  divisions:  the  cortex  (zona  parenchy- 
matosa),  a  narrow  peripheral  zone,  from  2-3  mm.  thick,  that  forms  the 
superficial  part  of  the  organ;  and  the  ^nedulla  (zona  vasculosa),  that  embraces 
the  deeper  and  more  central  remaining  portion  of  its  substance.  The  cortex 
alone  contains  the  characteristic  Graafian  or  egg-follicles  and  the  ova,  while 
the  medulla  is  distinguished  by  the  number  and  size  of  the  blood-vessels, 
especially  the  veins. 

The  Cortex. — Seen  in  sections  of  the  fimctioning  organ,  the  cortex 
appears  to  consist  chiefly  of  the  compact  ovarian  stroma,  a  modified  connec- 
tive tissue  composed  of  spindle-shaped  cells  and  fibrous  tissue.  The  stroma- 
/"<?/Zy  are  arranged  in  bundles  extending  in  all  directions  and,  hence,  are  seen 
cut  in  different  planes.  Immediately  beneath  the  modified  mesothelium,  the 
so-c3\\e.d.  germinal  epithelium,  that  covers  the  free  surface,  the  stroma  is  dis- 
posed with  greater  regularity  and  forms  a  narrow  compact  superficial  stratum, 
the  tunica  albiiginea,  in  which  the  ova  are  absent.  Within  the  subjacent 
and  looser  stroma  lie  the  most  characteristic  components  of  the  cortex,  the 
Graafian  or  egg-follicles,  that  represent  what  has  been  called  the  "  gland-sub- 

245 


246 


NORMAL   HISTOLOGY. 


stance''  (Stohr)  of  the  ovary.  The  follicles  are  in  different  stages  of  devel- 
opment, but,  for  the  most  part,  are  small,  inconspicuous  and  immature.  Cor- 
responding with  their  development,  the  egg-sacs  are  divided  into  primary, 
growing  and  inahwing  follicles.  In  general,  the  youngest  and  least  devel- 
oped lie  nearest  the  surface,  the  more  advanced  deep  and  towards  the  medulla, 
while  those  approaching  full  development  appear  as  huge  vesicles  that  may 


V^' 


p>^ 


'•''5=^ 

•W-"" 


Ovarian  x  --.. 
stroma    ,.^-' 


m 


Immature 

primary 

follicle 


Follicle  y^\-^^.~  ^  „  _ 
beginning  -f^'i^^^^-^cy^  C>«*V 
to  grow        ifJ/^J^^m.*c^  ■ 


Stratum 
granulosum 

I 
Ovum 


Theca 


Fig.  295. — Section  of  cortex  of  ovary  of  young  woman,  showing  primary  and  growing  follicles  within  the 

ovarian  stroma.     X  190. 


occupy  not  only  the  entire  thickness  of  the  cortex,  but  produce  marked  eleva- 
tion of  the  surface.  The  entire  number  of  ova,  as  estimated  from  the  ovaries 
of  a  seventeen-year-old  subject,  is  approximately  35,000  for  both  ovaries. 

The  immature  primary  follicles  are  microscopic  in  size  (40-60  /u.)  and,  in  the 
ovaries  of  young-  adults,  form  an  incomplete  and  scattered  single,  or  at  most  double 
layer.  Each  follicle  includes  the  centrally  situated  young  egg  or  ovulum,  surrounded 
by  a  single  row  of  flattened  epithelial  or  mantle  cells,  which  are  directly  lodged  within 
the  interstices  of  the  stroma-tissue.  The  ova  and  the  mantle  cells  are  derived  from 
the  proliferation  of  the  germifial  epithelium,  the  modified  mesothelium  covering  the 
germinal  ridge  on  the  median  surface  of  the  Wolffian  body.  Very  early  certain  cells 
are  distinguished  by  their  e.xceptional  size  and  large  clear  nuclei.  These  are  the 
primary  ova,  around  which  the  small  descendants  of  the  germinal  epithelium  become 
arranged  as  the  mantle  cells.  Soon  an  active  intergrowth  occurs  between  the  prolife- 
rating epithelium  and  the  invading  vascular  connective  tissue  of  the  Wolffian  body  that 
becomes  the  ovarian  stroma.  The  latter  increases  so  rapidly  that  the  primary  follicles, 
single  or  in  small  groups,  become  separated  by  augmenting  tracts  of  stroma-tissue. 

The  primary  ova,  approximately  spherical  and  40-50  /«  in  diameter,  may  remain 
for  years,  sometimes  from  early  infancy  to  advanced  age,  practically  unchanged,  until 
they  undergo  either  atrophy,  as  do  most  of  them,  or  further  growth  leading,  under 


THE   OVARIES. 


24: 


favorable  conditions,  to  the  production  of  mature  germ-cells.  Of  the  thousands  of 
primary-  ova  contained  within  the  ovaries  just  before  puberty,  only  comparatively  few- 
attain  perfection,  between  300-400  probably  being  the  maximum  number  liberated 
during  the  usual  period  of  sexual  activity.  When  enclosing  an  ovum  destined  for 
complete  development,  the  primary  follicle  enters  upon  a  period  of  active  growth,  the 
flat  mantle  cells  of  the  egg-sac  changing  into  a  single  layer  of  cuboid  epithelium. 

The  growing  follicles  are  distinguished  by  the  rapid  proliferation  of  their  cuboid 
epithelium  that  results  in  the  production  of  a  stratified /o//icu/ar  epithelium  surrounding 
the  ovum.     Outside  this  polygonal  epithelium,  the  stroma  condenses  into  a  connective 


Germinal  epithelium 


Primordial  ovum 


Ova 


Fk;.  296. — Section  of  developing  ovary  from  human  embryo,  showing  intergrowth  between  derivatives 
from  germinal  epithelium  and  stroma-tissue  from  Wolffian  body.     X  560. 

tissue  envelope,  the  theca,  which  subsequently  differentiates  into  an  outer  and  an  iiiuer 
tunic,  the  former  being  composed  of  concentrically  disposed  fibrous  tissue  and  the 
latter  of  round  or  spindle  cells  and  numerous  capillaries.  After  the  formation  of  the 
follicular  epithelium,  the  ovum  itself  begins  to  grow,  the  expansion  proceeding  uni- 
formly and  affecting  all  parts  of  the  cell,  including  the  nucleus  and  nucleolus.  It 
attains  its  maximum  diameter  long  before  the  follicle  reaches  full  growth.  Through 
the  activity  of  the  follicular  epithelium  the  e%%  becomes  invested  with  a  protecting 
envelope,  the  zoua  pellucida  or  radiata,  after  which  little  or  no  further  increase  in 
the  size  of  the  egg  occurs.  At  first  soHd,  the  growing  follicle  is  converted  into  a 
vesicle  containing  fluid  by,  at  first,  the  progressive  vacuolation  and  breaking  down  of 
the  cells  of  the  middle  layers  of  the  follicular  epithelium  and,  later,  by  the  transudation 
from  the  surrounding  blood-vessels.  This  fluid,  the  liquor  folliculi,  increases  to  such 
an  extent  that  it  soon  occupies  the  greater  part  of  the  expanding  egg-sac,  now  entering 
upon  its  final  stage  of  growth. 

The  maturing  follicles,  also  known  as  vesicular,  occupy  the  deeper  parts  of  the 
cortex  and  reach  to  the  medulla.  With  their  continued  expansion  they  appropriate 
more  and  more  of  the  cortex,  until  the  entire  thickness  of  the  latter  and,  sometimes, 
part  of  the  medulla  are  occupied  by  the  ripe  follicle,  which  just  before  its  rupture 
attains  a  diameter  of  from  1-2  cm.  and  models  the  free  surface  of  the  ovary  as  a  tense 
rounded  elevation.  After  rupture  and  liberation  of  the  ovum,  the  follicle  is  converted 
into  a  corpus  licteum. 


248 


NORMAL    HISTOLOGY. 


The  wall  of  the  ripe  follicle  consists  of  a  well-developed  capsule  or  theca,  a 
delicate  membrana  propria,  against  whose  inner  surface  lie  the  follicular  cells,  known 
as  the  stratum granulosum,  surrounding  the  space  filled  with  the  liquor  folliculi.    Oppo- 


Surface__ 
epithelium' 


Primary 
follicles" 


Theca  folliculi - 


Cumulus -^ 


Zona   pellucida-- 


Ovum  Cavity  filled  with  liquor  folliculi 

Fig.  297. — Section  of  ovary,  showing  partially  developed  Graafian  follicles.    X  90. 

site  the  point  where  rupture  takes  place,  the  stratum  granulosum  is  prolonged  into  a 
pedunculated  spherical  mass  of  epithelial  cells  that  projects  into  the  cavity.     This 

mass,  the  cumulus,  encloses  the 

j^      .  ovum  and  on  section  appears 

;^!  "^  I   (^     ^'1'  as  an  epithelial  ring,  the  corona 

(  radiata,  that  encircles  the  zona 

pellucida  and  the  ovum   and 
^  consists  of  two  or  three  layers 

<&''  of  radially  disposed  cells.     The 

membranous  zona  pellucida  is 
the  product  of  the  follicular  cells 
-  and,  therefore,  not  a  part  of  the 

-;."  '  ovum  proper.  It  sometimes  ex- 
hibits a  radial  striation,  hence 
is  often  called  zona  radiata, 
probably  due  to  penetrating 
processes  from  the  superim- 
posed epithelial  cells. 

The    human    ovum 

when  about  to  be  liberated 
from  the  Graafian  or  egg- 
folHcle  possesses  a  diameter 
of  from  .2-.  3  mm.      Its  cy- 
toplasm, the  vitellus  of  the 
older  writers,  exhibits  differ- 
entiation into  a  peripheral 
ooplasmic  and  a  central  deu- 
toplasmic  zone,  the   latter  being  dark  and  conspicuous  on  account  of  the 
irregular  refraction  of  the  enclosed  yolk-particles.     The  presence  of  a  distinct 
cell-wall,    or    egg-membrane,    in    the    human    ovum    is    doubtful,   although 


Fig.  29S. — Almost  mature  human  ovum  taken  from  fresh  ovary. 
Ovum,  wilh  germinal  vesicle  and  spot,  is  encircled  by  clear  zona 
pellucida,  which  is  surrounded  by  follicular  epithelium.    X  250. 

( Waldeyer.) 


THE   OVARIES.  249 

demonstrable  in  some  mammals.  The  spherical  egg-nucleus,  the  germinal 
vesicle,  lies  eccentrically  placed  within  the  deutoplasmic  zone  and  measures 
from  30-45  A'-  It  is  bounded  by  a  sharply  defined  nuclear  membrane  and 
contains  the  nucleolus,  ox  gerininal  spot  (4-8  //),  and  the  nuclear  reticulum. 

Coqjus  Luteum. — When  the  follicle  approaches  maturity,  the  inner  layer  of  the 
theca  becomes  the  seat  of  great  activity,  the  blood-vessels  increasing  and  the  cells 
undergoing  rapid  proliferation  and  extraordinary  growth,  the  enlarged  elements 
becoming  filled  with  a  yellowish  substance  and  transformed  into  lutein  cells.  Coin- 
cidently  the  follicular  epithelium    suffers   fatty  change  which  results  in  the    partial 


%iVV^v-  5V=i\\^--  ^..^s-  -  - 

Central 


«' 


,  %C>V^\  ^V^C^^s"^  ^->s?^!^,*K^?=:i^  connective  tissue 


-  Blood-vessels 

Fitj.  299.— Section  of  human  corpus  luteum.     X  70. 

disintegration  of  the  cumulus  and  freeing  of  the  ovum,  enclosed  by  the  cells  of  the 
corona  radiata,  into  the  cavity  of  the  follicle.  When  the  latter  ruptures  the  expulsion 
of  its  contents  is  followed  by  hemorrhage  into  the  cavity  of  the  follicle  and,  soon 
afterwards,  by  closure  of  the  opening  in  the  sac.  The  position  of  the  rupture  corre- 
sponds to  the  place  {stigma)  where  the  wall  of  the  follicle  is  most  distended  and  least 
vascular  and,  hence,  possesses  least  vitality  and  resistance.  The  rapid  proliferation 
and  growth  of  the  lutein  cells  produces  an  irregularly  plicated  wall  of  increasing 
thickness  that  encloses  the  remains  of  the  follicular  epithelium  and  invades  the  hem- 
orrhagic mass.  The  latter  is  gradually  absorbed  until  the  encroaching  projections  of 
lutein  cells  and  invading  vascular  connective  tissue  meet  and  the  cavity  of  the  follicle 
is  obliterated,  its  former  position  being  subsequently  indicated  by  a  central  core  of 
connective  tissue.  The  complex  thus  formed,  composed  of  lutein  cells  and  septa  of 
vascular  connective  tissue,  is  the  corpus  luteum. 

When  the  liberated  ovum  becomes  fertilized,  the  corpus  luteum  grows  to  huge 
dimensions  and  forms  a  conspicuous  oval  mass  that  may  approach  3  cm.  in  length  and 
occupy  a  considerable  part  of  the  entire  cortex.     When  associated  with  pregnancy  it 


250  NORMAL    HISTOLOGY. 

is  termed  the  corpus  liiteum  veruin.  If  impregnation  does  not  occur,  the  yellow  body, 
now  called  the  corpus  lutenni  spuriuni,  is  smaller  and  seldom  exceeds  from  1.5-2  cm. 
The  classic  distinction  of  "true"  and  "false"  has  no  anatomical  basis,  since,  apart 
from  size,  both  forms  are  structurally  identical.  The  assumption,  that  the  presence 
of  a  large  corpus  luteum  is  proof  of  pregnancy,  must  be  accepted  with  much  caution, 
since  yellow  bodies  of  unusual  size  are  sometimes  observed  in  ovaries  of  virgins. 
With  the  production  of  a  solid  corpus  luteum  and  the  absorption  of  the  blood, 
evidences  of  the  latter  remaining  for  a  long  time  as  hematoidin  crystals,  the  active 
role  of  the  lutein  cells  is  finished.  These  elements  now  lose  their  yellow  pigment 
{luieiti),  undergo  fatty  change  and  finally  entirely  disappear.  The  connective  tissue 
which  now  constitutes  the  entire  mass,  undergoes  hyaline  change,  becoming  clear 
and  nonfibrous,  while  the  aging  corpus   luteum   loses  its  former  appearance  and  is 

Meso- 
Blood-vessels      salpinx     Broad  ligament 

Mesovanuni  N  ~         '  ' 

Corpus  luteum 


Cortex 
Graafian  follicles 


Corpora  lutea 

r 


^  Sections  of  oviduct 
V"  .         Medulla 
^^ Remains  of  corpora  lutea 

Fig.  300. — Cross-section  through  ovary,  oviduct  and  part  of  broad  ligament.    X  4'. 

transformed  into  an  irregular  body,  light  in  color  and  sinuous  in  outline,  sometimes 
termed  the  corpus  albicans.  Although  gradually  absorbed,  the  latter  is  evident  for  a 
considerable  time,  especially  when  associated  with  pregnancy,  as  a  light  corrugated 
area  within  the  cortex. 

The  Medulla. — The  vascular  central  zone  of  the  ovary,  the  medulla, 
consists  of  comparatively  loose  stroma-tissue,  composed  of  irregularly  felted 
bundles  of  fibrous  tissue  rich  in  elastic  fibres,  supporting  the  vessels  and 
nerves.  In  the  mature  ovary,  with  the  exception  of  occasional  encroaching 
Graafian  follicles  that  are  ripening,  egg-sacs  are  not  found  within  the  medulla. 
On  the  other  hand,  it  contains  many  blood-vessels  some  of  which,  when  seen 
in  cross-section,  may  be  mistaken  by  the  inexperienced  observer  for  sections 
of  follicles.  The  larger  vessels  are  surrounded  by  considerable  tracts  of 
involuntary  muscle,  which  are  continuous  in  part  with  those  of  the  utero- 
ovarian  ligament,  through  the  hilum  and  mesovarium,  the  fold  of  peritoneum 
which  attaches  the  ovary  to  the  broad  ligament.  The  veins  are  particularly 
large  and  appear  in  sections  as  huge  blood-spaces  of  irregular  outline,  in 
consequence  of  their  tortuosity  and  plexiform  arrangement. 

The  blood-vessels  supplying  the  ovary  are  four  or  five  branches  from 
the  anastomotic  arch  formed  by  the  ovarian  and  uterine  arteries.  These 
branches,  the  arterice  proprice,  reach  the  medulla  through  the  hilum  as  closely 
grouped  tortuous  vessels.  On  gaining  the  interior  of  the  ovary,  each  stem 
divides  into  two  viedullary  or  parallel  arteries  that  proceed  directly  towards 
the  opposite  free  margin  of  the  organ,  lying  just  beneath  the  cortex  to  which 


THE   OVARIES. 


251 


they  distribute  cortical  branches  at  regular  intervals.  In  their  course  towards 
the  periphery  the  cortical  branches  supply  hundreds  of  follicular  twigs  to 
the  egg-sacs,  each  of  the  latter  being  provided  with  a  rich  vascular  network 


Blood- 


FiG.  501. — Section  of  medulla  of  ovar>'.  showing  numerous  blood-vessels  and  fibro-muscular 

stroma.     X  75- 

that  anastomoses  with  two  or  more  follicular  twigs.      At  the  periphery  of  the 
organ,  the  blood  within  the  cortical  arterioles  reaches  the  veins  through  an 
intervening  capillary  network.      The  veins  follow  the  general  arrangement  of 
the  arteries  in  the  cortex  and  medulla; 
the    pairs  of    parallel  veins,   however, 
do  not  unite    into    single   stems,   but 
emerge  from  the  hiluni  as  independent 
trunks. 

The  lymphatics  begin  in  the 
cortex  as  networks  of  spaces  within 
the  thecae  surrounding  the  enlarging 
follicles.  From  these  radicles  the 
larger  and  irregular  channels  enter  the 
medulla,  where  they  form  converging 
stems  that  follow  the  blood-vessels  and 
leave  the  hilum  as  7-9  trunks. 

The  nerves  supplying  the  ovary 
are  from  the  sympathetic  plexus  sur- 
rounding the  ovarian  artery  and  are 
composed,  for  the  most  part,  of  non- 
medullated  fibres.  They  accompany 
the  arteries  through  the  hilum  into 
the  ovary  and  are  distributed  chiefly  to  the  walls  of  the  blood-vessels,  around 
the  larger  of  which  the  terminal  plexuses  are  formed.  From  the  fairly  close 
cortical  ple.xus  twigs  pass  to  the  larger  follicles,  the  ultimate  relation  between 


Superficial 
anastomoses 


Follicular 
anastomoses 
Follicular 
branches 


Arteria  propria 


Ovarian  artery 


Ovarian 
veins 


302. — Diagram  illustrating  arrangement   of 
ovarian  blood-vessels.     (Clark.) 


252 


NORMAL    HISTOLOGY. 


the  latter  and  the  surrounding-  fibres  being  uncertain.  It  is  probable,  how- 
ever, that  the  fibrils  end  mostly  in  the  walls  of  the  follicular  blood-vessels, 
although  some  are  said  to  penetrate  to  the  follicular  epithelium.  The  exist- 
ence of  sympathetic  ganglia  within  the  medulla  has  not  been  established. 


RUDIMENTARY  ORGANS  REPRESENTING  FCETAL  REMAINS. 

In  the  male  the  Wolffian  body  and  its  duct  play  very  important  roles  in 
the  development  of  the  excretory  canal  for  the  sexual  gland,  while  the 
Miillerian  duct  remains  rudimentary.  In  the  female  the  converse  is  true,  the 
Miillerian  ducts  forming  the  excretory  canals- — the  oviducts,  the  uterus  and 
the  vagina — while  the  Wolffian  structures  are  of  secondary  importance  and 


Male 


GM 


Female 


Ur  CG  Pr 


Fig.  303. — Diagrams  illustrating  differentiation  of  two  sexes ;  derivates  from  Wolffian  body  are  red, 
those  from  Miillerian  duct  are  blue.  Male:  T,  testicle;  VE,  vasa  efferentia ;  GM,  globus  major;  VD, 
vas  deferens;  Pa,  paradidymis;  VA,  vas  aberrans ;  SV,  seminal  vesicle;  AT,  appendix  testis;  AE, 
appendix  epididymidis ;  B,  bladder;  PU,  prostatic  utricle;  Pr,  prostate;  Ur,  urethra;  CG,  Cowper's 
gland  ;  CC,  corpus  cavernosum  ;  R,  rectum  ;  RD,  renal  duct ;  K,  kidney.  Female  :  O,  ovary ;  Ov,  ovi- 
duct;  F,  fimbria  ;  U,  uterus  ;  V,  vagina  ;  DEp,  duct  of  epoophoron  ;  TEp,  tubules  of  epoophoron  ;  Po, 
paroophoron  ;  HM,  hydatid  of  Mofgagni ;  GD,  Gartner's  duct;  BG,  Bartholin's  gland;  C,  clitoris;  K, 
kidney;  R,  rectum.     {Modified  from  Wiedersheim..) 

give  rise  to  rudimentary  and  functionless  organs,  situated  chiefly  in  the 
vicinity  of  the  ovary  and  the.  Fallopian  tube  between  the  layers  of  the  broad 
ligament.  These  foetal  remains  include:  the  epoophoron,  Gartner' s  dud,  t\ie 
paroophoron,  and  the  vesicular  appendages. 

The  epoophoron,  also  called  the  parovarium  ox  organ  of  Rosenmuller, 
lies  between  the  layers  of  the  broad  ligament,  in  the  area  bounded  by  the 
ampulla  of  the  oviduct,  the  ovarian  fimbria  and  the  tubal  pole  of  the  ovary. 
It  is  flat,  triangular  or  trapezoidal  in  outline,  and  measures  from  2-2. 5  cm. 
in  length.  It  consists  of  from  eight  to  twenty  narrow  wavy  tubules,  the 
ductuli  transversi,  which,  beginning  with  closed  and  slightly  dilated  ends, 
diverge  from  the  vicinity  of  the  hilum  of  the  ovary  and  join,  almost  at  right 
angles,    a  common  chief  duct  that  lies  close  and  parallel  to  the  oviduct, 


RUDIMENTARY   ORGANS.  253 

bearing  to  the  smaller  tubules  the  relation  of  the  back  of  a  comb  to  its  teeth. 
The  transverse  tubules  are  the  remains  of  the  sexual  tubules  of  the  Wolfifian 
body;  the  common  canal,  the  ductus  longitudinalis,  is  closed  at  both  ends 
and  represents  a  persistent  portion  of  the  Wolfifian  duct.  The  longitudinal 
duct  may  be  interrupted  and  connected  with  the  tubules  in  groups,  or,  on 
the  other  hand,  it  may  be  prolonged  as  Gartner's  duct  far  beyond  its  usual 
length.  In  the  child,  the  transverse  tubules  (.3-4  mm.  in  diameter}  usually 
possess  a  lumen,  but  later  in  life  they  may  undergo  partial  or  com.plete 
occlusion  and  may  be  the  seat  of  cysts.  The  walls  of  the  tubules  and  duct 
consist  of  a  fibrous  coat,  which  sometimes  contains  bundles  of  unstriped 
muscle,  lined  with  a  single  layer  of  epithelial  cells  that  var}'  in  form,  from 
low  cuboid  to  columnar  and  occasionally  bear  cilia. 

Gartner's  duct  results  from  the  more  or  less  extensive  persistence  of 
portions  of  the  Wolffian  duct  that  usually  disappear  by  the  end  of  foetal  life; 
it  is,  therefore,  a  continuation,  direct  or  interrupted,  of  the  longitudinal  duct 
of  the  epoophoron.  When  complete,  as  it  very  e.xceptionally  is,  the  duct 
continues  from  the  epoophoron,  along  the  oxiduct  and  the  side  of  the  uterus, 
to  the  lower  end  of  the  vagina.  Such  extensive  persistence  is  unusual, 
Gartner's  duct  being  mostly  limited  to  the  lower  part  of  the  body  and  the 
upper  cervix  of  the  uterus.  The  canal  is  lined  with  a  single  layer  of 
columnar  epithelium  and  beset  with  uncertain  lateral  diverticula,  which  may 
be  short  branched  tubules  resembling  glands.  Accumulations  of  secretion 
within  the  duct  or  its  diverticula  may  lead  to  the  production  of  cysts. 

The  paroophoron  is  an  inconspicuous  rudimentary  organ,  distinct  at 
birth  but  usual) v  disappearing  after  the  second  year,  that  lies  within  the 
broad  ligament  between  the  epoophoron  and  the  uterus.  It  consists  of  a 
small  irregularly  round  group  of  blind  canals,  lined  with  a  single  layer  of 
columnar  epithelium,  that  often  resemble  the  Wolfifian  tubules,  which  struct- 
ures, in  fact,  they  represent.  A  second  group  of  similar  rudimentary  tubules 
lies  lateral  to  the  epoophoron.  It  is  this  group,  perhaps,  that  should  be 
regarded  as  the  paroophoron  proper  and  the  homologue  of  the  paradidymis 
in  the  male.      The  tubules  may  be  the  seat  of  cysts. 

The  vesicular  appendages  include  the  small  vesicles  or  hydatids 
attached  to  the  broad  ligament  by  longer  or  shorter  stalks.  They  comprise 
two  groups,  the  one  being  represented  by  the  conspicuous  long-stalked 
hydatids  of  Morgagni  and  the  other  by  the  smaller  vesicles,  varying  in  form 
and  size,  connected  by  short  stems.  The  hydatid  of  Morgagni  is  a  spheri- 
cal or  pyriform  thin-walled  sac,  that  contains  a  clear  fluid  and  usually 
measures  from  4-8  mm.  in  diameter.  The  vesicle  is  attached  by  a  slender 
stalk,  from  1.5-4  cm.  long,  to  the  anterior  surface  of  the  broad  ligament 
and  is  continuous  with  the  upper  blind  end  of  the  longitudinal  duct  of  the 
epoophoron.  The  hydatid  consists  of  a  fibrous  coat,  lined  by  a  single  layer 
of  columnar  epithelium,  and  covered  externally  by  a  delicate  prolongation 
of  peritoneal  tissue.  The  small  vesicles  are  attached  to  the  anterior  surface 
of  the  broad  ligament,  usually  over  the  epoophoron.  The  origin  and  mor- 
phological significance  of  the  vesicular  appendages  have  occasioned  much 
discussion,  but  it  may  be  accepted  as  established  that  the  hydatid  of 
Morgagni  is  derived  from  the  upper  end  of  the  Wolfifian  duct,  and  is, 
therefore,  the  equivalent  of  the  appendage  of  the  epididymis.  The 
smaller  vesicles,  which  correspond  in  structure  with  the  larger  one, 
probably  owe  their  origin  to  the  distention  and  elongation  of  some  of  the 
transverse  tubules  of  the  epoophoron,  and,  hence,  are  derivations  of  the 
Wolfifian  tubules. 


254 


NORMAL    HISTOLOGY. 


THE   OVIDUCTS. 

The  oviduct  or  Fallopia7i  tube^  also  called  the  tuba  uterincE,  is,  in 
principle,  the  excretory  canal  of  the  sexual  gland,  since  it  conveys  the  ova 
liberated  from  the  ovary  to  the  uterus,  into  which  it  opens.  The  relation 
between  the  ovary  and  its  duct  is  exceptional,  in  that  these  organs  are  not 
continuous  but  only  in  apposition,  the  ova  liberated  from  the  ruptured 
Graafian  follicles  finding  their  way  into  the  expanded  end  of  the  oviduct. 
This  canal,  one  on  each  side  of  the  body,  lies  within  the  free  border  of  the 
broad  ligament  and  extends  laterally  from  the  uterus  to  the  ovary,  in  relation 
with  the  mesial  surface  of  which  it  ends  after  repeated  windings.      The  entire 


=5^^^^^^=^3:*=§^Kj^, 


Mucosa 


Epithelium 


Lumen 


Fig.  304. — Transverse  section  of  oviduct  near  outer  end  of  ampulla.     X  35- 


length  of  the  tube  is  about  11.5  cm.  (4^  in.),  its  diameter  increasing  from 
3-4  mm.  at  the  istlvmis,  next  the  uterus,  to  from  6-8  mm.  at  the  outer  limit 
of  the  ampulla,  where  the  canal  suddenly  expands  into  the  terminal  trumpet- 
shaped  infimdibiihmi.  The  mucous  membrane  lining  the  oviduct  is  thrown 
into  longitudinal  folds,  which  become  progressively  more  marked  towards 
the  outer  end,  so  that  cross-sections  of  the  ampulla  present  a  lumen  of 
complex  outline  owing  to  the  projection  of  primary  and  secondary  plications 
(Fig.  304).  At  the  irregularly  notched  or  fimbriated  margin  of  the  infun- 
dibulum,  the  mucous  lining  of  the  tube  is  directly  continuous  with  the 
peritoneum.  The  exceptional  relation  of  the  tubal  mucosa  to  the  serous 
membrane,  this  being  the  only  place  in  the  body  w^here  a  mucous  tract 
directly  communicates  with  a  serous  sac,  is  a  persistence  of  the  similar 
relation  of  the  embryonal  Miillerian  duct,  from  which  the  oviduct  is  directly 
derived. 


THE   UTERUS.  255 

The  wall  proper  of  the  oviduct  consists  of  the  ifiucous  and  tnusailar 
coats  and  is  embedded  within  the  loose  connective  tissue  of  the  broad  liga- 
ment {tunica  adventitia) ,  and  surrounded  by  the  serous  coat,  which  com- 
pletely invests  the  duct  with  the  exception  of  the  narrow  interval  through 
which  the  tubal  vessels  and  nerves  pass.  The  wall  is  thickest  and  firmest 
in  the  isthmus,  less  so  in  the  ampulla,  and  thinnest  and  most  relaxed  in  the 
infundibulum  and  fimbriae.  The  mucous  coat  is  thrown  into  longitudinal 
folds,  which  in  the  ampulla  attain  much  complexity  and  in  transverse  sec- 
tions appear  as  branching  villus-like  projections. 

The  mucous  membrane  is  covered  by  a  single  layer  of  columnar 
epithelium  provided  with  cilia,  whose  current  is  directed  towards  the  uterus, 
thus  favoring  the  progress  of  the  ova  along  the  tube  but  retarding  the  ascent 
of  the  spermatozoa.  The  tunica  propria  consists  of  bundles  of  fibrous  tissue, 
is  rich  in  cells  and  directly  continuous  with  the  intermuscular  connective 
tissue.  Its  deepest  layer  often  contains  irregular  strands  of  muscle-bundles,  \ 
which  suggest  a  muscularis  mucosae.  The  muscular  coat,  upon  which 
the  mucosa  rests  without  the  intervention  of  a  submucous  layer,  is  most 
robust  towards  the  uterus  and  thinnest  at  the  infundibulum.  It  includes  an 
inner  circular  and  an  outer  longitudinal  layer  of  unstriped  muscle.  At  the 
isthmus,  where  the  firmness  of  the  tubal  wall  depends  chiefly  on  the  muscular 
coat,  the  circular  layer  is  the  thicker  (.5-1  mm.)  and  the  longitudinal  one 
is  incomplete;  towards  the  infundibulum  the  reverse  is  true,  the  longitudinal 
layer  being  better  developed  and  the  circular  muscle  reduced  to  .  2  mm.  or 
less.  The  surrounding  fibrous  tissue,  sometimes  described  as  a  distinct  coat, 
the  tunica  adventitia,  blends  with  the  fibro-elastic  stroma  of  the  investing 
peritoneum,  which  may  be  regarded  as  the  serous  coat.  Since  these 
structures  consist  of  the  usual  connective  tissue  and  mesothelial  elements  of 
peritoneum  (page  175),  a  special  description  is  unnecessary. 

The  blood-vessels  supplying  the  oviduct,  derived  from  the  tubal 
branches  of  the  uterine  and  ovarian  arteries,  gain  the  wall  of  the  tube  along 
the  nonperitoneal  tract  and  break  up  into  numerous  branches  between  the 
outer  and  inner  muscular  layers,  from  which  capillaries  pass  to  the  muscular 
tissue  and  to  the  mucous  membrane.  The  veins  begin  within  the  mucosa 
and  join  the  many  intermuscular  channels,  from  which  tributaries  pass  to  the 
subserous  meshwork.  The  lymphatics,  after  emerging  from  the  wall  of 
the  tube  within  which  they  begin  as  irregular  spaces  between  the  fibrous 
bundles  of  the  muscular  coat,  form  three  or  four  stems  that  accompany  the 
blood-vessels.  The  nerves  are  numerous  and  chiefly  sympathetic  fibres 
from  the  ovarian  and  the  uterine  plexus.  Within  the  subserous  tissue  they 
form  a  peritubal  ple.xus,  from  which  twigs  penetrate  the  wall  of  the  canal  to 
supply  chiefly  the  involuntary  muscle,  some  fibres  entering  the  mucosa. 

THE   ITERUS. 

The  uterus  or  womb  is  a  hollow  pear-shaped  muscular  organ,  receiving 
the  oviducts  above  and  opening  into  the  upper  part  of  the  vagina  below,  in 
which  the  fertilized  ovum  is  retained  and  develops,  and  from  which  the 
resulting  foetus  is  expelled  at  the  completion  of  gestation.  It  measures 
about  7  cm.  in  length,  of  which  the  lower  2.5  cm.  constitutes  the  neck,  or 
cervix,  and  the  remainder  the  body;  its  greatest  breadth  is  about  4  cm.  and 
its  thickness  2.5  cm.  The  convex  upper  extremity  of  the  organ  is  known 
as  \.\\e  fundus.  Of  the  two  surfaces,  the  anterior  is  only  partially  and  the 
posterior  almost  completely  covered  with  peritoneum. 


256 


NORMAL    HIST0L0(;Y. 


The  uterine  wall  is  thickest  at  the  fundus  and  posterior  aspect  of  the 
body,  where  it  measures  1-1.5  cm.,  and  somewhat  thinner  (8-9  mm.)  at  the 
entrance  of  the  oviducts  and  in  the  cervix.  It  comprises  three  coats — the 
mucous,  the  muscular,  and  the  serous.  The  mucous  coat,  or  endometrium, 
is  .  5—1  mm.  thick  and  consists  of  a  tunica  propria  of  fibrous  tissue,  contain- 
ing a  large  number  of  colorless  blood-cells,  and  the  surface  epithehum.  The 
latter  is  a  single  layer  of  columnar  cells,  about  28  ij.  high,  that  in  their  typical 
condition  possess  cilia  producing  a  current  towards  the  cervix.     The  cilia. 


Gland 
opening-  on 
mucous  sur- 
face 


-  Muscular 

tissue 


f=r-~  —  Blood-vessel 


Fig.  305.— Section  of  mucous  membrane  of  uterus,  showing  glands  cut  in  various  planes.     X  4°- 

however,  are  neither  always  present  nor  uniformly  distributed,  since  they 
are  lost  during  menstruation  and  often  present  only  in  patches  (Gage). 

The  uterine  glands  are  simple  tubular  or  slightly  bifurcated  wavy 
invaginations,  lined  with  a  single  layer  of  ciliated  columnar  cells  resembling 
tho.se  covering  the  adjacent  uterine  mucosa.  They  are  distributed  at  fairly 
regular  intervals  and  extend  the  entire  thickness  of  the  mucosa,  their  tor- 
tuous blind  extremities  lying  close  to  the  subjacent  muscle,  since  a  submucosa 
is  wanting.  At  the  orifices  of  the  oviducts,  the  uterine  mucosa  becomes 
thinner,  the  epithelium  lower,  and  the  glands  shorter  and  fewer,  until  they 
finally  disappear,  glands  being  absent  in  the  tubal  mucous  membrane. 

The  mucous  membrane  of  the  cervical  canal  is  somewhat  thicker  and 
denser  than  that  lining  the  body  of  the  uterus.  The  single-layered  columnar 
cells  vary,  in  some  places  being  taller  (40-50  //)  than  those  lining  the  body. 


THE    UTERUS. 


257 


in  others  lower  and  more  cuboidal.  In  addition  to  tlie  usual  tubular  crypts, 
which  although  larger  resemble  those  in  the  body,  the  cervical  glands  include 
wide  di\erticulated  mucous  follicles  producing  a  clear  peculiarly  tenacious 
secretion,  \\nien  the  latter  is  retained,  the  glands  are  converted  into  cysts  that 
a])pear  as  minute  \'esicles  between  the  characteristically  converging  folds  of  the 
cerxical  mucosa  and  were  formerly  known  as  the oz'/t/a  Nabothi.  The  abrupt 
transition  of  the  columnar  epithelium  of  the  cervical  canal  into  the  stratified 
squamous  cells  covering  the  vaginal  portion  of  the  uterus  takes  place,  before 
pregnancy  has  occurred,  at  the  inner  border  of  the  external  orifice.  After 
the  changes  incident  to  pregnancy  have  affected  the  uterus,  this  transition  lies 


Fold  of  mucosa 


Recess  bet 
folds,  coiitai 

secretion 


ween      "'^:'-''^^.         %'M  %\--' <^:^\'^^'--'->il^^:^-  IV' i  -.-' • 
■etion      »...\T  -.f-.^  -'^XS^'    V-t-  ^  ^> s->  'V:- ■ « ••'.^^  S,  -  -^  —  ■    •  •. 


Cen'ical  glands 


Muscle 


■;>■  »"  ," 


Blood-vessel 


Fig.  306.- 


-Longitudinal  section  of  cervical  mucous  membrane,  showing  glands  opening  into  recesses 
between  the  plicae.     X  50. 


higher,  approximately  the  lower  half  of  the  cervical  canal  then  being  clothed 
with  the  squamous  epithelium.  The  change  of  the  cervical  mucosa  into  that 
lining  the  body  of  the  uterus  is  gradual  and  without  definite  demarcation. 

The  muscular  coat,  or  myomctriian,  is  composed  of  bundles  of 
involuntary  muscle  arranged  with  little  regularity;  it  is  possible,  however, 
to  distinguish  two  general  strata — a  robust  innej'  layer,  in  which  the  bundles 
possess  a  circular  disposition,  and  a  thin  imperfect  outer  layer,  whose 
component  bundles  are  for  the  most  part  longitudinal.  The  innermost 
bundles  nf  the  circular  layer  are  oblique  and  longitudinal  and  sometimes 
described  as  a  distinct  submucous  layer.  The  thick  circular  layer,  the  chief 
component  of  the  myometrium,  is  distinguished  by  the  number  and  size  of 
the  venous  channels  that  traverse  the  intermuscular  connective  tissue;  hence 
its  designation  as  the  stratiun  vasctdare.  At  the  orifices  of  the  oviducts  and 
the  internal  cer\-ical  opening,  the  disposition  of  the  muscle-bundles  suggests 
a  sphincter.  The  longitudinal  muscle  is  most  distinct  over  the  fundus  and 
body,  being  unrepresented  in  the  cervical  segment.  Here  the  circular  and 
oblique  bundles  are  intermingled  with  a  considerable  quantity  of  fibro-elastic 
tissue,  an  arrangement  conferring  greater  resistance  and  hardness  upon  the 

17 


258  NORMAL    HISTOLOGY. 

cervix.  The  longitudinal  muscle-bundles  are  continued  beyond  the  uterus 
into  the  oviducts  and  the  broad,  round,  ovarian  and  utero-sacral  ligaments. 
The  component  iibre-cells  of  the  uterine  muscle  vary  in  form,  in  some  places 
being  short  and  broad  and  in  others  long  and  fusiform. 

The  serous  coat,  or  perimetrium,  continuous  laterally  with  the  peri- 
toneal investment  of  the  broad  ligament,  is  closely  adherent  to  the  uterine 
muscle  over  the  fundus  and  adjacent  parts  of  the  anterior  and  posterior  surfaces. 

The  blood-vessels  approach  the  uterus  between  the  layers  of  the  broad 
ligament.  On  gaining  the  muscular  coat,  the  larger  branches  divide  into 
twigs  that  penetrate  the  outer  layer  of  the  myometrium  and  within  the  cir- 

Longitudinal  muscle 

,-^  Blood-vessels 


Attachment/ 

of  broad''         / ;  ___=«^ 

ligamentV         '    ,,  x^^S^isg^^^^^^^^':' — --thY^ 


>.»^?=^ 


~~  Circular  muscle 

Mucous  membrane     \ 

V  , ■  Peritoneum 

Longitudinal  muscle ^^^^ — ^^^ 

Fig.  307. — Transverse  section  of  uterus  through  the  body.    X  i?^. 

cular  muscle  break  up  into  tortuous  branches  which  in  part  pass  to  the 
mucous  membrane  and,  in  conjunction  with  the  large  veins,  confer  a  highly 
vascular  character  to  the  stratum.  Within  the  mucosa,  the  capillaries  sur- 
round the  glands  and  form  a  network  beneath  the  epithelium.  The  veins 
begin  in  the  mucosa,  but  within  the  middle  of  the  muscular  coat  form  large 
tortuous  channels,  sections  of  which  appear  as  conspicuous  irregular  spaces 
between  the  muscle-bundles. 

The  lymphatics  within  the  mucosa  are  represented  by  a  network  of 
lymph-spaces,  from  which  stems  pass  through  the  muscular  coat  to  join  the 
close-meshed  subserous  network  of  larger  lymphatics.  The  efferent  trunks 
pursue  various  courses  and  communicate  with  the  lymphatics  of  the  neigh- 
boring organs — vagina,  rectum,  ovaries,  and  oviducts. 

The  nerves  of  the  uterus  are  abundant  and  include  both  sympathetic 
and  spinal  fibres,  nonmedullated  and  medullated.  Since  their  chief  destina- 
tion is  the  involuntary  muscle  and  blood-vessels,  the  nonmedullated  fibres 
are  associated  with  minute  terminal  ganglia  from  which  the  terminal  filaments 
pass  to  the  myometrium.  Other  fibres  reach  the  mucosa,  within  which  a 
close  subepithelial  plexus  is  formed,  fibrils  probably  entering  the  epithelium. 

Changes  During  Menstruation  and  Pregnancy. — Although  hberation  of  a  mature 
ovum  may  occur  at  any  time,  in  the  vast  majority  of  cases  ovulation  and  menstruation 
are  synchronous  processes,  the  uterine  changes  occurring  regularly,  every  twenty- 
eight  days,  only  when  the  ovaries  are  functionally  active.  In  anticipation  of  the 
possible   reception   of  a   fertilized   ovum,  the   uterine  mucous   membrane  becomes 


THE   VAGINA.  259 

swollen,  excessively  vascular  and  hypertrophied,  with  conspicuous  enlargement  of 
the  subepithelial  blood-vessels  and  the  glands.  The  resulting  thickened  and  modified 
mucosa,  now  from  3-6  mm.  thick,  offers  a  soft  velvety  surface  favorable  for  the 
implantation  of  the  embryo-sac.  Should  this  occur,  the  hypertrophy  proceeds,  and 
tlie  lining  of  the  uterus  is  converted  into  the  deciduce  and  takes  an  important  part  in 
the  formation  of  the  placenta.  If,  on  the  contrary,  fertilization  does  not  occur,  the 
proliferative  processes  are  arrested  and  the  hypertrophied  mucosa,  now  called  the 
decidua  incustrualis,  enters  upon  regression.  Incidental  to  the  latter  are  subepithelial 
extravasation  and  rupture  and  partial  destruction  of  the  epithelium,  followed  by  the 
characteristic  discharge  of  blood.  While  usually  the  destruction  of  the  mucosa  is 
limited  to  the  epithelium,  it  is  probable  that  at  times  the  superficial  layer  of  the  sub- 
jacent tissue  is  involved. 

During  pregnancy  the  most  conspicuous  changes  are  occasioned  by  the  growth 
necessary  to  accommodate  the  rapidly  augmenting  volume  of  the  uterine  contents,  by 
the  provision  of  an  adequate  source  of  nutrition  and  protection  for  the  fcEtus,  and  by 
the  development  of  an  efficient  contractile  apparatus  for  the  expulsion  of  the  same. 
The  enormous  increase  depends  especially  upon  the  hypertrophy  of  the  muscular 
coat,  which  during  the  first  half  of  pregnancy  becomes  greatly  thickened,  but  later 
thinner  and  membranous  owing  to  stretching.  The  increase  results  from  both  the 
growth  of  the  previously  existing  muscle-cells  and,  during  the  first  half  of  pregnancy, 
the  development  of  new  muscle  elements.  The  individual  cells  may  increase  tenfold 
in  length  and  measure  between  .4-5  mm.  During  the  first  five  months,  the  mucous  mem- 
brane of  tlie  body  also  becomes  greatly  hypertrophied,  in  places  attaining  a  thickness 
from  7-10  mm.  The  glands  and  blood-vessels,  particularly  the  arteries,  enlarge  and, 
within  the  specialized  area,  are  concerned  in  the  formation  of  the  placenta.  The 
cervical  mucosa  takes  no  direct  part  in  the  formation  of  the  deciduae,  although  it 
thickens  and  is  the  seat  of  enlarged  glands  that  secrete  the  plug  of  mucus  that  for  a 
time  occludes  the  mouth  of  the  uterus.  After  the  termination  of  pregnancy,  the 
uterus  enters  upon  a  period  of  involution  and  repair,  the  excessive  muscular  tissue 
undergoing  degeneration  and  absorption  and  the  lacerated  mucosa  regeneration,  the 
latter  process  being  completed  in  from  five  to  si.x  weeks. 

THE   VAGINA. 

The  vagina  is  a  flattened  muscular  tube,  lined  with  mucous  membrane, 
that  extends  from  the  genital  cleft  enclosed  by  the  labia  below  to  the  uterus 
above.  Its  walls,  from  2-3  mm.  thick,  include  a  mucous  and  a  muscular 
coat,  supplemented  externally  by  a  less  definite  fibrous  tunic. 

The  mucous  coat  consists  of  stratified  squamous  epithelium  and  a 
fibro-elastic  tunica  propria,  exceptionally  rich  in  veins  and  colorless  blood- 
cells  and  beset  with  numerous  conical  papillae  that  encroach  upon  the 
overlying  epithelium,  but  do  not  model  the  free  surface.  Although  nor- 
mally moistened  by  a  thin  mucous  secretion  of  acid  reaction,  the  vagina  is 
devoid  of  glands.  Small  lymph-nodules  are  scattered  through  the  mucosa, 
especially  in  the  upper  part  of  the  canal.  The  hymen,  the  membranous  fold 
partly  occluding  the  vaginal  orifice,  consists  of  a  basis  of  vascular  fibrous 
tissue  covered  by  a  prolongation  of  the  mucous  membrane. 

The  muscular  coat,  which  supports  the  mucous  membrane  without 
the  intervention  of  a  distinct  submucous  layer,  is  composed  of  bundles  of 
unstriped  muscle  arranged,  although  not  with  precision,  as  an  inner  circular 
and  an  outer  longitudinal  layer.  The  latter  is  best  developed  over  the 
anterior  vaginal  wall,  from  which  strands  of  muscular  tissue  are  continued 
into  the  urethro-vaginal  septum.  Behind,  bundles  are  prolonged  into  the 
recto-vaginal  partition;  above,  the  vaginal  muscle  is  continuous  with  that  of 
the  uterus  and  below  penetrates  the  perineal  body.  Within  the  conspicuous 
elevations,  the  columnce  riigaruin,  marking  the  vaginal  wall,  both  mucous  and 


26o 


NORMAL    HISTOLOGY. 


muscular  coats  are  thickened,  the  elevations  acquiring  somewhat  the  character 
of  erectile  tissue  owing-  to  the  abundance  of  \-eins  intermingled  with  irregularly 
disposed  muscle-bundles.  The  fibrous  coat,  outside  the  muscular,  is  com- 
posed of  closely  felted  bundles  of  tibrous  tissue  and  plentiful  elastic  fibres. 

The  blood-vessels  supplying  the  vagina,  deri\ed  from  several  sources, 
form  a  netM-ork  between  the  mucous  and  muscular  coats  from  which  some 
twigs  pass  to  the  muscle  and  others  enter  the  mucosa,  where  they  break  up 


Surface  iijjn 
epithelium  Wi^V 


Blood-  - 

vessels 


Circulai  ■ 
muscle 


Longitudinal 
muscle 


_y 


Fig.  30S. — Section  of  wall  of  vagina,  showing  the  rugae  cut  across.    X  80. 

into  a  capillary  network.  The  veins  are  very  numerous,  and  unite  into  a 
close  plexus  within  the  muscular  tunic,  from  which  large  emergent  trunks 
extend  along  the  sides  of  the  canal. 

The  lymphatics  are  numerous  and  represented  by  an  exceptionally  close 
network  within  the  mucosa,  one  less  dense  within  the  muscular  coat  and  a 
superficial  network  o\'er  the  exterior  from  -which  the  larger  main  efterent 
stems  arise. 

The  nerves  of  the  vagina  are  chiefly  sympathetic  efterents,  associated 
with  minute  ganglia  as  they  tra\-erse  the  fibrous  coat,  for  the  supply  of  the 
blood-vessels  and  involuntary  muscle.  The  sensory  fibres  distributed  to  the 
mucous  membrane  lining  the  upper  part  of  the  \'agina  are  meagre,  the  pudic 
nerves  endowing  the  mucosa  of  the  lower  third  of  the  canal  with  greater 
sensibility.  Sensory  nerve-endings  of  different  kinds  have  been  obser^'ed 
Avithin  the  mucous  membrane. 


THE  EXTERNAL  ORGAN.S. 

The  labia  majora  are  rounded  cutaneous  folds,  the  homologue  of  the 
scrotum,  the  int*egument  covering  the  outer  surface  being  thick,  dark  hued 
and  beset  with  large  hair-follicles.  That  covering  the  medial  surface  is 
much   more  delicate  in  texture,    with  few  and  minute   hairs.      Sweat-  and 


thp:  extp:rnal  organs. 


261 


sebaceous  glands  are  numerous.  In  addition  to  the  investment  of  skin, 
each  labium  majus  contains  a  layer  of  subcutaneous  fat,  between  which 
and  the  integument  lies  a  thin  stratum  of  involuntary  muscle,  tunica 
dartolahialis^  continued  ft)rwards  from  the  dartos  of  the  perineum.  The 
centre  of  the  labium  is  occupied  by  a  fairly  well  defined  mass  of  fat,  the 
corpus  adiposum,  that  is  separated  from  the  subcutaneous  tissue  by  a 
delicate  fibro-elastic  membrane. 

The  labia  minora  or  nymphae  are  thin  folds  of  delicate  skin,  con- 
tinuous with  the  greater  labia  at  the  bottom  of  the  interlabial  groove,  on  the 
one  hand,  and  with  the  mucous  membrane  lining  the  vestibule,  on  the  other. 
Although  both  surfaces  are   covered  with  integument,   the  protection  and 


Central  fat  body 


Labium  majus 


Labium  minus 


Inner  surface 


Sebaceous  glands  on  external 
cutaneous  surfaces 


Interlabial  groove 


Fig.  309. — Section  across  the  labia  of  young  child.     X  18. 

contact  with  the  vaginal  secretions  to  which  the  median  aspect  of  the  fold  is 
subjected,  modify  the  skin  on  the  inner  side  so  that  it  assumes  the  color  and 
appearance  of  a  mucous  membrane.  The  line  of  transition  into  the  ves- 
tibular mucosa  follows  the  medial  attachment  of  the  fold.  The  absence  of 
mucous  glands  and  the  presence  of  sebaceous  follicles  on  both  surfaces  are 
differential  characteristics  of  skin  as  contrasted  with  the  adjacent  mucous 
membrane.  In  addition  tc  the  two  cutaneous  layers,  the  nymphae  are 
composed  of  an  intermediate  stratum  of  loose  connective  tissue,  rich  in 
blood-vessels  and  bundles  of  unstriped  muscle,  that  resembles  erectile  tissue. 
Hairs  and  fat  are  entirely  wanting  on  the  labia  minora,  but  sebaceous  and 
sweat-glands  are  plentiful  after  the  first  few  years. 

The  vestibule,  the  space  enclosed  by  the  nymphae,  is  lined  with 
mucous  membrane  covered  by  stratified  squamous  epithelium  and  containing 
many  mucous  glands.  Close  to  the  posterior  margin  of  the  urethral  orifice, 
or  on  the  papilla  that  usually  marks  this  opening,  lie  the  small  apertures  of  the 
paraurethral  ducts.  These  canals,  also  knowm  as  the  tubes  of  Skene  and  from 
1-2  cm.  long,  lead  into  smaller  groups  of  branched  tubules,  which  are  regarded 
as  the  homologues  of  the  prostatic  tubules.     The  diicts  are  lined  with  stratified 


262 


NORMAL   HISTOLOGY. 


squamous  epithelium  for  a  short  distance  from  the  vestibule,  the  remainder 
of  the  passage  and  its  subdivisions  being  clothed  with  columnar  cells. 

The  glands  of  Bartholin,  the  largest  of  the  vestibular  and  the  homo- 
logues  of  the  bulbo-urethral  (Cowper's)  glands,  are  two  small  organs,  1-1.5 
cm.  in  length,  situated  one  on  either  side  of  the  vaginal  orifice.  They  are 
tubo-alveolar  mucous  in  type  and  produce  a  whitish  viscid  secretion.  The 
small  component  lobules  are  separated  by  considerable  tracts  of  fibro- 
muscular  tissue  and  lined  with  columnar  epithelium  containing  many  mucus- 
bearing  cells.  The  lobular  ducts  unite  to  form  the  single  excretory  canal, 
which  is  beset  with  minute  mucous  follicles.  The  main  duct,  sometimes 
provided  with  an  ampullary  dilatation,  is  lined  with  columnar  epithelium 
until  near  its  termination,  where  the  epithelium  becomes  stratified  squamous 
to  correspond  with  that  of  the  vestibule. 

The  clitoris,  the  homologue  of  the  penis,  possesses  in  reduced  and 
modified  form  the  chief  components  of  the  male  organ.  It  consists  essen- 
tially of  two  miniature  corpora  cavernosa  and  an  imperfectly  developed  and 
cleft  corpus  spongiosum,  known  as  the  bulbus  vestibuli.  The  latter  consists 
of  two  converging  elongated  masses  of  cavernous  tissue — a  complex  of 
tortuous  veins  and  fibro-muscular  tissue.  The  glans  and  carvernous  bodies 
repeat,  although  in  less  typical  manner,  the  histological  details  described  in 
connection  with  the  corresponding  parts  of  the  male  (page  239). 


Excretory  duct 


THE   MAMMARY   GLANDS. 

Although  morphologically  modified  cutaneous  glands  and  developed  in 
both  sexes,  the  functional  importance  of  the  mammary  glands,  or  tnammcB, 
in  the   female  entitles  them   to   be   regarded   as  organs   accessory  to   the 

reproductive  apparatus. 
Each  mamma,  or 
breast,  comprises  a 
group  of  some  twenty 
individual  and  separate 
glands,  opening  on  the 
nipple  by  independent 
ducts,  that  collectively 
constitute  the  secreting 
organ,  the  corpus  mam- 
mcz^  as  distinguished 
from  the  enveloping  fat 
and  areolar  tissue.  Prior 
to  the  changes  incident 
to  pregnancy,  the  secre- 
tory tissue  is  relatively 
meagre  and  overshad- 
owed by  the  fat-laden 
connective  tissue  in 
which  the  still  rudi- 
mentary alveoli  are  em- 
bedded. 

The  corpus  mam- 
mae consists  of  from  15-20  or  more  flattened  pyramidal  lobes,  radially 
disposed,  with  the  bases  directed  towards  the  periphery  and  the  excretory 
canals,  the  ladiferozis  ducts,  converging  towards  the  nipple  upon  which  they 


^-f-  Involuntary 
\        muscle 


Iveoli 


Interalveolar 
stroma 


Fig.  310. — Section  of  mammary  gland  before  lactation.    X  17°. 


THE   MAMMARY   GLANDS. 


263 


open.  Each  lobe  is  subdivided  by  connective  tissue  into  several  lobules^ 
which  in  turn  are  made  up  of  the  ultimate  divisions  of  the  secreting  tissue, 
the  alveoli.  The  walls  of  the  latter  consist  of  a  membrana  propria,  lined,  in 
the  resting  condition,  by  a  double  layer  of  cells.  Those  next  the  membrana 
propria  are  flat  and  probably  muscular  in  nature,  thus  emphasizing  the 
resemblance  between  the  mammary  and  sweat-glands.  The  inner  cells,  the 
secretory  elements,  are  cuboid  or  low  columnar. 

During  lactation,  the  alveoli  become  greatly  enlarged  and  distended 
and  the  intervening  connective  tissue  correspondingly  reduced,  so  that  the 
alveoli  are  pressed  closely  together.  The  cytoplasm  of  the  cells  engaged  in 
the  production  of  milk  contains  minute  oil  droplets,  which,  as  they  increase 
in  size,  displace  the  nucleus  towards  the  membrana  propria  and  project  into 
the  lumen  of  the  alveolus,  being  separated  from  the  latter  by  only  a  thin 
protoplasmic  envelope.  With  the  rupture  of  the  cells  the  oil  drops  escape 
into  the  albuminous  fluid,  additionally  secreted  by  the  cells,  that  occupies 
the  alveolus.  After  liberation  of  the  oil  droplets,  the  epithelial  cells  are 
much  reduced;  after  a 
time,  however,  they 
again  become  the  seat 
of  renewed  secretory  ac- 
tivity, the  accumulation 
of  fat  and  the  production 
of  milk.  Destruction 
of  the  secreting  cells, 
therefore,  does  not  take 
place. 

The  excretory 
ducts  begin  as  the  small 
canals  into  which  the  al- 
veoli open  and,  at  first, 
resemble  the  terminal 
compartments  of  the 
gland,  being  lined  with 
a  delicate  stratum  of 
striped  muscle,  upon 
which  rests  a  simple 
cuboidal  epithelium.  Within  the  ladiferoiis  ducts,  formed  by  the  junction  of 
the  smaller  canals,  the  cuboid  cells  are  succeeded  by  columnar  ones.  On 
approaching  the  base  of  the  nipple,  beneath  the  colored  areola,  each  milk- 
duct  enlarges  into  a  spindle-form  ampulla  or  sinus  lactiferus,  from  10-12 
mm.  long  and  about  half  as  wide,  that  serves  as  a  temporary  reservoir  for 
the  secretion  of  the  gland.  Beyond  the  ampulla  the  duct  narrows  (2  mm.), 
passes  into  the  nipple,  and  ends,  after  ascending  the  latter  parallel  with  the 
other  ducts,  in  a  minute  orifice  (.5-. 7  mm.)  at  the  summit  of  the  nipple. 
Just  before  terminating,  the  epithelium  lining  the  duct  assumes  the  stratified 
squamous  character  of  the  adjacent  epidermis. 

The  skin  covering  the  areola  and  nipple,  delicate  but  more  or  less 
pigmented,  contains  well  marked  bundles  of  unstriped  muscle,  whose  con- 
tractions cause  the  nipple  to  become  prominent  and  erect.  Within  the 
areola,  this  contractile  tissue  forms  a  layer,  in  places  almost  2  mm.  thick, 
that  encircles  the  base  of  the  nipple  and  extend^  into  its  substance  as  a 
muscular  network  through  which  the  milk-ducts  pass.  Deeper  longitudinal 
strands  of  unstriped  muscle  occupy  the  axis  of  the  nipple. 


Fig.  311. 


i^~lnteralveolar 
'         septum 


—Section  of  mammary  gland  during  lactation,  showing  dis- 
tended alveoH  lined  with  fat-bearing  cells.     X  170. 


264  NORMAL    HISTOLOGY. 

Over  both  areola  and  nipple  the  skin  is  provided  with  large  sebaceous 
glands,  the  secretion  of  which  is  increased  during  lactation  and  serves  as 
protection  during  nursing.  Sweat-glands  are  wanting  over  the  nipple  but 
large  and  modified  in  the  periphery  of  the  areola.  The  surface  of  the  latter 
is  modelled,  especially  towards  the  close  of  pregnancy,  by  low  rounded 
elevations  that  mark  the  position  of  the  subcutaneous  areolar  glands  of 
Montgomery.  The  latter  are  rudimentary  accessory  masses  of  glandular 
tissue,  from  1-4  mm.  in  diameter,  and  correspond  in  general  structure  with 
the  mammary  glands.  Their  ducts  open  by  minute  orifices  on  the  surface 
of  the  areola. 

Milk. — The  fully  established  secretion  of  the  mammary  gland  is  an 
emulsion,  the  fat-globules  being  suspended  in  a  clear  colorless  watery  plasma. 
The  composition  of  human  milk  includes  over  86  per  cent,  of  water,  about 
3  of  albuminous  substances,  5.3  of  fat,  5  of  sugar,  and  less  than  i  per  cent, 
of  salts.  The  chief  morphological  constituents  of  milk  are  the  mUk-globides, 
as  oil  droplets  liberated  from  the  alveolar  cells  are  called;  these  \ary  in  size 
from  the  most  minute  spherules  to  those  ha\'ing  a  diameter  of  3-5  p.  or  more. 


CC    o 


A  B 

3 


O 


C 


c 


(^       :^ 


c  or  H^oo,    o^^    ^!:  °        ^,         -^^ 


■  <^   o    ^ 

o 

Fig.  312. — Human  milk  ;  A,  ordinary  secretion  ;  B,  showing  colostrum  corpuscles  and  oil-drops.     ^<  400. 

Their  average  number  per  cubic  millimeter  is  something  over  one  million 
(Bouchut).  Whether  the  milk-globules  are  enclosed  within  extremely  thin 
envelopes  of  casein  is  uncertain.  It  is  probable  that  the  fat-particles  are  not 
produced  within  the  gland-cells,  but  are  taken  up  and  temporarily  stored  by 
their  cytoplasm.  A  variable  number  of  migratory  leucocytes,  more  or  less 
filled  with  fat-particles,  are  usually  present  in  milk. 

During  the  last  weeks  of  gestation  and  for  two  or  three  days  after  its 
termination,  the  breasts  contain  a  clear  watery  secretioh,  known  as  colostrum, 
that  differs  from  milk  in  possessing  relatively  little  fat  and  numerous  conspicu- 
ous bodies,  the  colostrum  corpiiscles.  The  latter  are  spherical,  but  may 
be  irregular  in  outline,  and  measure  from  12—18  //,  although  they  may  attain 
a  diameter  of  more  than  40  //.  The  corpuscles  are  composite  bodies  and 
consist  of  a  complex  of  leucocytes  greatly  distended  with  fat-particles  and  of 
modified  ah'eolar  epithelial  cells.  Their  cytoplasm  is  markedly  granular  and 
often  of  a  yellowish  or  reddish-yellow  tint.  They  appear  after  lactation  has 
ended  and  may  be  expressed  from  the  regressing  gland  for  months  or,  in 
exceptional  cases,  for  even  years.  Quite  commonly  the  mammary  glands  in 
both  sexes,  during  the  first  few  clays  after  birth,  yield  a  secretion  resembling^ 
colostrum,  popularly  known  as  "witch-milk." 

At  birth  the  gland  is  represented  by  the  lactiferous  ducts  with  their 
ampulke,  the  smaller  collecting  ducts  and  the  rudimentary  alveoli.  The 
mammae  remain  small  and  immature  during  childhood  until  the  approach  of 


THE   MAMMARY  GLANDS.  265 

sexual  maturity,  when  they  increase  in  size  and  rotundity  in  consequence 
chiefly  of  the  deposition  of  fat.  The  full  development  of  the  true  gland  is 
deferred  until  the  occurrence  of  pregnancy,  when  active  proliferation  and 
increase  of  the  gland-tissue  takes  place  in  preparation  for  its  activity  as  a 
milk-producing  organ.  After  lactation  has  ended,  the  mammae  undergo 
involution,  the  glandular  tissue  being  reduced  and  returning  to  a  condition 
resembling  that  before  pregnancy.  With  the  recurrence  of  the  latter,  the 
gland  again  enters  upon  a  period  of  renewed  growth  and  j^reparation,  to  be 
followed  in  time  by  return  to  the  resting  condition,  in  which  the  amount  of 
glandular  tissue  is  inconspicuous.  After  cessation  of  menstruation,  the 
mammary  gland  gradually  decreases  in  size,  and  in  ad\'anced  age  the  corpus 
mammae  may  be  reduced  to  a  fibrous  disk  in  which  gland-tissue  is  almost,  if 
indeed  not  entirely,  wanting. 

The  blood-vessels  supplying  the  mammary  gland  in  addition  to  their 
distribution  to  the  skin  and  more  superficial  parts  of  the  breast,  send  deeper 
twigs  to  the  glandular  tissue  which  break  up  into  capillary  networks  surround- 
ing the  alveoli.  During  lactation  the  vascular  supply  is  materially  increased. 
The  veins  from  the  corpus  mamnice  join  the  superficial  \'essels  and,  in  the  main, 
follow  the  arteries.  Within  the  areola,  the  subcutaneous  veins  forma  ple.xus 
that  encircles  the  nipple  and  receives  its  blood.  The  lymphatics  are 
exceptionally  numerous  and  important.  The  deeper  ones  lie  within  the 
interlobular  connecti\'e  tissue  and  pass  towards  the  surlace,  where  they  join 
the  rich  subareolar  network.  With  the  exception  of  a  few  trunks  that  follow 
the  perforating  arteries  and  become  ef^erents  of  the  internal  mammary  lymph- 
nodes,  the  lymphatics  of  the  breast  form  two  or  three  large  trunks  that  pass 
to  the  axillary  nodes.  The  nerves  supplying  the  glandular  tissue  are  chiefly 
svmpathetic  fibres,  some  ending  in  the  blood-vessels  and  others  forming 
plexuses  upon  the  membrana  propria  of  the  alveoli,  a  few  fibrils  terminating 
between  the  secretintr  cells. 


THE  CENTRAL  NERVOUS  SYSTEM. 

The  central  nervous  system  includes  the  spinal  cord  and  the  brain. 
In  principle  these  are  the  walls  of  the  primary  neural  tube,  modified  by 
unequal  growth  and  expansion,  which  even  after  acquiring  definite  relations 
enclose  the  remains  of  the  tube,  as  represented  by  the  brain-ventricles  and 
the  central  canal  of  the  cord.  In  contrast  to  the  spinal  segment  of  the 
neural  tube,  which  always  remains  a  relatively  simple  cylinder,  the  cephalic 
segment  early  differentiates  into  the  cerebral  vesicles,  marked  flexure  occur- 
ring coincidently  at  certain  points.  From  the  sinuously  bent  cephalic  seg- 
ment are  developed  the  fundamental  parts  of  the  brain,  while  from  the 
relatively  straight  spinal  segment  proceeds  the  development  of  the  spinal 
cord,  during  which  process  growth  and  differentiation  convert  the  originally 
thin-walled  tube  into  an  almost  solid  cylinder,  the  minute  central  canal  alone 
remaining  as  the  representative  of  the  once  conspicuous  lumen. 

THE  SPINAL  CORD. 

The  spinal  cord,  or  medidla  spinalis,  is  that  part  of  the  central  nervous 
system,  or  cerebro-spinal  axis,  which  lies  within  the  vertebral  canal.  After 
removal  of   its  protecting  membranes  and  the  attached  root-fibres  of   the 


Posterior  median  septum 


Posterior  column 

Posterior  rootTfurrow 


Substantia 
gelatinosa  T^ 

Rolandi         f^ 

Caput  cornu 


Cervix  cornu  T 


Lateral  cornu  "',-'"">;' 


Basis  cornu 


Caput  cornu 


Posterior 
root-fibres 


Lateral  column 


^       Central  canal 
in  gray 
commissure 


Anterior  median  fissure        Anterior  column 


Anterior  white  commissure 


Fig.  313. — Cross-section  of  child's  cord  through  thoracic  region,  showing  arrangement  of  gray  and  white 
matter  and  subdivision  of  the  latter  into  columns.     X  13. 

spinal  nerves,  the  spinal  cord  is  seen  to  be  a  flattened  cylinder,  so  that  the 
antero-posterior  diameter  is  always  less  than  the  transverse  one;  its  outline 
in  cross-sections,  therefore,  is  not  circular  but  more  or  less  oval.      Its  width, 
266 


THE   SPINAL   CORD. 


>67 


moreover,  is  not  uniform  on  account  of  two  fusiform  swellings,  the  cervical 
and  luDibar  enlargemeyits,  associated  with  the  origin  and  reception  of  the 
large  nerves  supplying  the  limbs.  Where  least  expanded,  opposite  the 
middle  of  the  thoracic  spine,  the  cord  measures  8  mm.  in  its  sagittal  and 
ID  mm.  in  its  transverse  diameter.  Through  the  cervical  enlargement  these 
respective  dimensions  are  9  mm.  and  14  mm.,  and  through  the  lumbar 
swelling  they  are  8.5  mm.  and  12  mm. 

Cross-sections  of  the  spinal  cord  (Fig.  313)  show  it  to  be  imperfectly 
divided  into  symmetrical  halves  by  a  narrow  cleft,  the  a^iterior  median  fissure, 
in  front  and  a  partition,  the  posterior  median  septum,  behind.  Further,  the 
cord  is  seen,  even  with  the  unaided  eye,  to  be  composed  of  an  irregular 
H -shaped  core  of  gray  substance  enclosed  by  a  mantle  of   white  matter. 


>w 


Nerve-fibres  of  white  matter  Anterior  root-fibres 

Fig.  314.— Portion  of  anterior  horn  of  gray  matter,  showing  multipolar  nerve-cells  and  root-fibres.    X  120. 

The  latter,  in  each  half  of  the  cord,  is  partially  subdivided  into  three  general 
tracts  by  the  lines  along  which  the  root-fibres  of  the  spinal  nerves  are  attached. 
The  dorsal  root-line  of  the  sensory  fibres  is  marked  by  a  slight  furrow,  the 
postero-lateral  sulcus,  that  lies  from  2.5-3.5  mm.  lateral  to  the  posterior 
median  septum.  The  ventral  root-line,  marking  the  emergence  of  the 
anterior  (motor)  fibres,  is  much  less  evident  on  account  of  the  scattered 
manner  in  which  these  root-fibres  make  their  exit.  In  this  manner  three 
longitudinal  tracts,  the  columns  of  the  cord,  are  marked  of?  on  each 
side — the  posterior  between  the  median  septum  and  the  postero-lateral  sulcuSj 


268  NORMAL    HISTOLOGY. 

the  lateral  between  the  dorsal  and  ventral  root-hnes,  and  the  anterior  \)q!on^&\\ 
the  ventral  root-line  and  median  fissure.  The  conventional  division  between 
the  anterior  and  lateral  columns,  however,  is  largely  artificial,  since  neither 
superficially  nor  internally  is  there  a  definite  demarcation  between  these 
tracts.  In  the  lower  cervical  and  upper  thoracic  cord,  the  posterior  column 
is  subdi\ided  by  the  superficial paraviedian  sulciis  and  a  septum  of  neuroglia 
into  two  wedge-shaped  tracts,  of  which  the  median  and  smaller  is  the  funic- 
7iIhs  gracilis  or  tract  of  Gall  and  the  lateral  and  larger  is  the  funiculus 
cuneatus  or  tract  of  Burdach. 

The  Gray  Matter. — Within  each  half  of  the  cord  the  gray  matter 
forms  a  comet-shaped  area,  the  broader  end  of  which  lies  in  front  and  the 
narrow  one  behind,  with  the  concavity  directed  laterally.  The  convex 
mesial  surfaces  of  the  areas  of  the  t^'o  sides  are  connected  by  a  transverse 
band  of  gray  matter,  the  gray  commissure,  that  extends  across  the  mid- 
line and  encloses  the  minute  central  canal  of  the  cord.  The  connecting 
band  is  subdi\ided  by  the  canal  into  the  posterior  and  the  anterior  gray 
commissure,  which  lie  behind  and  in  front  of  the  tube  respectix-ely.  The 
posterior  median  septum  reaches  the  dorsal  surface  of  the  gray  commissure, 
but  the  ventral  margin  of  the  latter  is  separated  from  the  bottom  of  the 
anterior  median  fissure  by  an  intervening  bridge  of  white  matter,  the  anterior 
■white  commissure,  which  connects  the  anterior  columns  and  provides  an 
important  pathway  for  fibres  passing  from  one  side  of  the  cord  to  the  other. 

Each  crescent  of  gray  matter  is  divided  conventionally  into  three 
parts:  the.  ajiterior  s.nd  posterior  corjiua,  the  ventral  and  dorsal  extremities 
of  the  crescent  that  project  beyond  the  line  of  the  transverse  gray  commis- 
sure, and  the  pars  intermedia,  that  connects  the  cornua  and  receives  the 
commissure.  The  two  horns  differ  markedly  and,  although  varying  in 
details  at  different  levels,  retain  their  distinctive  features  throughput  the 
cord.  The  anterior  cornu,  or  columna  grisea  anterior,  is  short  thick  and 
rounded  and  separated  from  the  surface  by  a  considerable  layer  of  white 
matter,  through  which  the  ventral  root-fibres  pass  to  their  points  of  emerg- 
ence from  the  cord.  The  blunt  tip  of  the  anterior  horn  is  known  as  the 
caput  cornu  and  the  dorsal  portion,  by  which  it  joins  the  commissure  and 
the  pars  intermedia,  as  the  basis  cornu.  The  posterior  cornu,  ox  columna 
grisea  posterior,  is  relatively  long  narrow  and  pointed  and  extends  almost  to 
the  postero-lateral  sulcus.  The  tip  or  apex  of  the  dorsal  horn  is  formed  of  a 
A-shaped  stratum,  the  substantia  gelatinosa  Rolandi,  that  appears  lighter 
and  somewhat  less  opaque  than  the  subjacent  caput  cornu,  which  it  covers  as 
a  cap.  The  slightly  contracted  ventral  portion  of  the  posterior  horn  is  the 
cervix  cornu. 

The  fairly  sharp  demarcation  between  the  gray  and  white  matter  is 
interrupted  along  the  lateral  border  of  the  crescent  by  prolongations  of  gray 
matter  into  the  adjacent  lateral  column  (Fig.  322).  The  subdivisions  oi 
these  processes  unite  into  a  reticulum  of  gray  matter,,  the  meshes  of  which 
are  occupied  by  longitudinally  coursing  nerve-fibres,  the  whole  interlacement 
being  known  as  \k\e  formatio  reticularis.  This  structure  is  best  developed  in 
the  upper  cervical  region,  where  it  is  a  conspicuous  network  filling  the  recess 
between  the  pars  intermedia  and  the  neck  of  the  posterior  cornu.  In  the 
thoracic  region,  the  formatio  reticularis  is  condensed  into  a  compact  lateral 
projection  of  gray  matter,  the  lateral  cornu,  or  columna  lateralis. 

The  histological  components  of  the  gray  matter  include  the  nerve- 
cells,  the  nerve-fibres  and  the  supporting  neuroglia.  Of  these  the  most 
distinctive   elements  are    the    multipolar    nerve-cells,    which    lie    embedded 


THE   SPINAL   CORD.  269 

within  a  complex  sponge-like  matrix  formed  by  the  feltwork  of  various  proc- 
esses— dendrites,  axones  and  collaterals — from  mostly  other  neurones,  the 
supporting-  neurogliar  reticulum  and  the  blood-vessels.  In  two  localities, 
immediately  around  the  central  canal  and  capping  the  dorsal  cornua,  the 
grav  matter  varies  in  appearance  and  constitution  and  exhibits  the  modifica- 
tions peculiar  to  the  central  and  Rolandic  substantia  gelatinosa. 

The  nerve -cells  of  the  anterior  cornu  are  multipolar,  the  cell-bodies 
appearing  irregularlv  polygonal  in  cross-sections  and  fusiform  in  longitudinal 
ones,  and  measure  from  65-135  //.  in  diameter  unless  unusually  small,  when 
their  diameter  may  \-ary  from  30-80  /'..  The  most  conspicuous  and  important 
elements  of  this  region  of  the  gray  matter  are  the  motor  radicular  cells, 


Fig.  315. — Isolated  root-cell  from  anterior  horn  of  cord  of  calf ;  a,  axone.     X  210. 

whose  axones  pass  from  the  apex  of  the  cornu,  tra\-ersc  the  white  matter 
(meanwhile  becoming  medullated),  and  emerge  from  the  cord  as  the  axis- 
cylinders  of  the  efi'erent  (motor)  root-fibres  of  the  spinal  nerves.  These 
cells  possess  from  three  to  ten  dendritic  processes,  which  radiate  in  various 
planes,  divide  dichotomously  and  finally  end  in  terminal  arborizations  that 
may  reach  the  posterior  horn  and  other  parts  of  the  gray  matter.  In  contrast 
to  the  robust  dendrites  beset  with  spines,  the  axone  is  slender  smooth  and 
directly  continuous  with  a  root-fibre  of  a  spinal  nerve.  With  the  exception 
of  delicate  collaterals,  which  may  be  wanting,  the  axone  is  unbranched. 
Each  ner\e-cell  possesses  a  spherical  or  ellipsoidal  nucleus  ( 10-20  // ),  enclosed 
by  a  distinct  nuclear  membrane,  and  usuallv  a  single  nucleolus,  exceptionally 
more  than  one.  In  addition  to  accumulations  of  deeply  stainirrt^-  tigroid  sub- 
stance, the  cvtoplasm  contains  brounish-yellow  pigment  granules,  often  in  the 
\icinity  of  the  implantation  cone  from  which  the  axone  springs. 

In  addition  to  the  foregoing  radicular  cells,  the  anterior  cornu  contains  other 
nervous  elements,  the  commissural  and  strand-cells.  The  commissural  cells  occur 
cliiefly  within  the  median  part  of  the  anterior  horn  and  resemble  in  size  and  form  the 
radicular  cells,  but  possess  smaller  nuclei.  Their  axones  traverse  the  anterior  white 
commissure  to  gain  the  opposite  anterior  cohimn,  in  which  they  divide  T-like  into 
ascending  and  descending  fibres,  or  undivided  turn  brainwards.  The  strand-cells, 
variable  in  form  and  generally  smaller  than  the  root-cells,  are  only  sparingly  repre- 


270  NORMAL    HISTOLOGY. 

sented  in  the  anterior  cornu.  Their  axones  usually  pass  to  the  anterior  column  of  the 
same  side,  but  sometimes  an  axone  divides,  one  fibre  crossing  by  way  of  the  anterior 
white  commissure  to  the  opposite  anterior  column,  while  the  other  passes  to  the  col- 
umn of  the  same  side. 

Although  the  nerve-cells  of  the  anterior  cornu  are  widely  scattered,  they  are  not 
uniformly  distributed  through  the  gray  matter,  but  are  collected  into  more  or  less 
definite  groups  that  recur  in  consecutive  sections.  It  is  evident,  therefore,  that  the 
cell-groups  are  not  limited  to  a  single  plane,  but  are  continuous  as  cell-columns  for 
longer  or  shorter  stretches  within  the  gray  matter.     The  grouping  of  the  nerve-cells 


Cells  of 
-substantia 
gelatinosa 

Posterior  horn 
'cells 


'<> 


Accessory  dorso- 
■  lateral  group 


Dorso-lateral 
-group. 


\Ventro-lateral  group 

Median  group 
Fig.  316. — Half-section  of  lower  cervical  cord,  showing  grouping  of  the  nerve-cells.    X  20. 

of  the  anterior  cornu  includes  two  general  collections,  a  viesial  group,  containing 
many  commissural  cells,  and  a  lateral  group,  composed  chiefly  of  ventral  radicular 
cells.  These  collections,  moreover,  vary  in  extent  and  definition  in  different  parts  of 
the  cord,  and,  where  well  marked,  are  often  made  up  of  more  than  a  single  aggrega- 
tion of  cells.  This  is  particularly  true  of  the  lateral  collection,  in  which  an  anterior 
and  a  posterior  subdivision  are  recognized  as  the  ventro-lateral  and  the  dorso-lateral 
group  (Fig.  316).  The  mesial  collection,  situated  within  the  ventral  angle  of  the  horn, 
is  sometimes,  but  much  less  clearly,  divisible  into  a  veiitro-tnesial  and  a  dorso-mesial 
group,  the  latter  being  variable  and  at  many  levels  wanting. 

The  nerve-cells  of  the  posterior  cornu  are  neither  as  large  nor  as 
regularly  disposed  as  the  anterior  horn  cells.  Only  in  one  locality,  along 
the  median  border  of  the  base  of  the  posterior  cornu,  are  they  collected  into 
a  distinct  tract — the  column  of  Clarke;  elsewhere,  they  are  scattered  without 
order  throughout  the  gray  matter  of  the  dorsal  horn.      Li  a  general  way, 


THE   SPINAL   CORD. 


271 


however,  they  may  be  divided  into:  (i)  the  cells  of  Clarke's  cohi?nn,  (2) 
the  cells  of  the  substantia  gelatinosa  Rolatidi,  and  (3)  the  iiiner  cells  of  the 
caput  cornu. 

The  cells  of  Clarke's  column  form  a  conspicuous  collection  that  occupies  the 
mesial  border  of  the  base  of  the  posterior  horn  (Fig.  317),  and  correspond  to  an 
elevation  on  the  surface  of  the  gray  matter.  The  cells  are  about  50  fi  in  diameter, 
polygonal  in  outline  and  possess  many  richly  branched  dendrites  that  radiate  chiefly 
within  the  limits  of  the  group.  The  axones  course  ventrally  for  some  distance  before 
bending  outward  toward  the  lateral  column  within  which,  as  the  constituent  fibres  of 
the  direct  cerebellar  tract  (fasciculus  cerebello-spinalis),  they  turn  brainwards. 

The  nerve-cells  of  the  substantia  gelatinosa  include  innumerable  small  stellate 
elements,  less  frequently  fusiform  or  pear-shaped,  that  measure  only  from  6-20  fi, 
although    exceptionally   larger.     Their    numerous    short    dendrites    are    irregularly 


White  matter  of 
posterior  column 


? 


'^  K>^^ 


Cells  of  Clarke  s  Lolumn 

Substantia  gelatinosa  centralis 


Central  canal 
Fig.  317.— Part  of  cord,  showing  cells  of  Clarke's  column  in  base  of  posterior  horn.    X  no. 

disposed  and  branched.  The  axones  are  continued  partly  into  the  adjacent  white 
matter  of  the  posterior  column,  where  they  divide  into  ascending  and  descending 
limbs,  and  partly  into  the  gray  matter,  in  which  they  run  as  longitudinal  fibres.  The 
marginal  cells  are  fusiform  or  pyramidal  and  larger  (35-55  /')  and  occupy  the  border 
of  the  substantia  gelatinosa.  Their  axones  traverse  the  substantia  gelatinosa  and 
probably  continue,  for  the  most  part,  within  the  lateral  colunm,  although  some  enter 
the  posterior.  Many  of  the  nerve-cells  within  the  substantia  gelatinosa  were  formerly 
regarded  as  glia  cells,  an  exceptionally  large  amount  of  neuroglia  being  credited  to 
this  substance;  instead  of  such  being  the  case,  this  region  of  the  gray  matter  is 
relatively  poor  in  neuroglia  and  numerically  rich  in  nerve-cells. 

The  inner  cells  of  the  posterior  cornu,  intermingled  with  numerous  nervous 
elements  of  small  size,  are  triangular  or  spindle-shaped  and  usually  measure  about 
50  n\  they  are,  therefore,  larger  than  the  ordinary  cells  of  the  substantia  gelatinosa. 
Their  axones  pass  mostly  into  the  lateral  column  of  the  same  side;  some,  however, 
have  been  traced  into  the  posterior  or  anterior  columns  of  the  same  and,  rarely,  into 


272  NORMAL    HISTOLOGY. 

the  opposite  anterior  column.  A  few  cells  of  type  II— those  whose  axones  are  not 
prolonged  into  ner\e-fibres  but  soon  break  up  into  elaborate  end-arborizations 
confined  to  the  gray  matter— are  found  within  the  gray  matter  of  the  posterior  cornua. 
The  nerve-cells  of  the  pars  intermedia,  the  gray  matter  which  connects  the  horns 
and  lies  opposite  the  gray  commissure,  may  be  divided  broadly  into  two  classes,  the 
lateral  and  the  middle  cells,  that  occupy  respectively  the  outer  border  and  the  more 
central  area  of  this  part  of  the  gray  matter.  Those  of  the  first  class,  or  intermedio- 
lateral  cells,  are  associated  with  the  formatio  reticularis  and  the  lateral  horn  and, 
hence,  are  also  called  the  group  or  column  of  the  lateral  horn.  Where  there  is  a 
distinct  lateral  horn,  as  in  the  thoracic  region,  the  cells  occupy  this  projection,  but 
elsewhere  lie  within  the  base  of  the  gray  network.  The  cells  are  multipolar  or 
fusiform,  from  15-45  ^w  in  diameter,  and  provided  with  a  variable  number  of  dendrites. 
The  axones  pass  directly  into  the  lateral  column  and  become  ascending  or  descending 
fibres;  a  few  axones  may  enter  the  anterior  column  of  the  same  side.  The  cells  of 
the  second  class,  the  iufennediate  cells,  are  irregularly  disposed  and  only  in  the  upper 


, Posterior  median  septum 

Paramedian  septum 
subdividing  posterior  column 


Lateral  column 


Anterior  column 
Fig.  318.— Transverse  section  of  spinal  cord,  showing  general  arrangement  of  neuroglia.     X  9. 

part  of  the  cord  present  a  fairly  distinct  middle  grou]x  They  are  polygonal  or 
fusiform,  small  in  size,  and  proxided  with  irregular  dendrites.  The  axones  are 
continued  chiefly  within  the  lateral  column  of  the  same  side,  although  some  pass  to 
the  anterior  column  and  a  few  probably  cross  to  the  opposite  side. 

A  few  isolated  nerve-cells  are  usually  to  be  found  within  the  white  matter  in  the 
vicinity  of  the  more  superficially  placed  cell-columns.  These,  the  so-called  outlying 
cells,  are  regarded  as  elements  displaced  from  their  usual  position  during  the  dift'er- 
entiation  and  growth  of  the  white  and  gray  matter.  Similar  displacement  sometimes 
aftects  the  cells  of  the  spinal  ganglia,  which  then  may  be  found  within  the  cord. 

The  neuroglia  of  the  gray  matter  is  everywhere  present  as  a 
delicate  reticulum  supporting  the  nerve-cells  and  fibres.  The  structure  of 
neuroglia  having  been  described  (page  70),  the  special  features  of  its 
arrangement  may  be  noted.  The  feltwork  of  neuroglia  fibrils  within  the 
gray  matter  is,  in  general,  more  compact  than  within  the  white  matter  and, 
Further,  somewhat  denser  at  the  periphery  than  in  the  deeper  parts  of  the 
gray  core.  There  is,  however,  no  sharp  boundary  between  the  supporting 
tissue  of  the  white  and  gray  tracts,  since  numerous  glia  fibrils  extend  out- 
wards from  the  framework  of  the  gray  substance  to  be  lost  between  the  nerve- 
fibres  of  the   adjoining   columns.      This  feature   is  marked   in  the  anterior 


THE   SPINAL    CORD.  273 

cornu,  where  the  gha  hbrils  form  septa  of  considerable  thickness  that 
diverge  into  the  surrounding  white  columns;  moreover,  the  conspicuous 
processes  of  the  formatio  reticularis  and  the  projecting  lateral  horn  consist 
largely  of  neuroglia.  The  larger  nerve-cells  and  their  more  robust  processes 
are  ensheathed  by  interlacements  of  neuroglia  fibrillae.  The  amount  of 
neuroglia  varies  in  the  several  parts  of  the  posterior  horn;  thus,  the  extreme 
apex  consists  almost  entirely  of  glia  tissue,  although  within  the  substantia 
gelatinosa  Rolandi  the  number  of  glia  fibres  is  unusually  meagre.  Within 
the  caput  and  remaining  parts  of  the  posterior  horn,  the  neurogliar  elements 
are  similar  in  quantitv  and  disposition  to  those  in  the  anterior  cornu. 

Substantia  gelatinosa  centralis  is  the  name  given  to  a  zone  of 
peculiar  translucency  that  immediately  surrounds  the  central  canal.  This 
annular  area  consists  of  modified  neuroglia  in  which  radial  ependymal  fibres, 

continued  from  the  ependymal  cells  lining  the 

^_-  -— —   -      ^  central  canal,   are  interwoven  with  circularly 

p:°  ^  ,^  disposed  neuroglia  fibrillae,  the  whole  being  a 

/''C /^  ^  .^         ,     "  compact  stratum  interspersed  with  an  unusual 

'   '  ^ '  -^If"      '  number  of  glia  cells.      In  marked  contrast  to 

,  the  substantia  gelatinosa  which  caps  the  pos- 

1^        ^  '  terior   horns,    that    surrounding   the     central 

'Wv<^;s*  ■S-^nT^  '    '        canal  contains   no    nerve-cells   but   only  glia 

*^-r~i:^  Vlu'-^^^    ■-        elements. 

"        ^1  2^'^'  The  nerve-fibres  of  the  gray  matter 

r^*"  '  constitute  a  considerable  part  of  the  intricate 

^  :^-  ;  groundwork  in  which  the  nerve-cells  are  em- 

^     '  bedded.     The  fibres,  seen  traversing  the  gray 

,.  ,vnv><^  -'  matter  in  all  directions,  are  the  processes  con- 

tributed by  neurones  situated  at  the  same,  dif- 
'  J         ferent  or  even  remote  levels,  and  include:  (i) 
""^  :^^,.$^'^Z''''  the  collaterals  and  terminal  stems  of  the  dor- 

sal root-fibres  that  enter  the  gray  matter;  (2) 
Fig,  3i9.-Centrai  canal  and  sur-     terminal  fibres  of  the  descending  tracts  that 

roundina:    substantia   gelatinosa    cen-  ,   .  ,      .  .  ,       ,  ,      "^ 

trails  fiom  child's  cord;  the  canal  is     end  HI  relation  With  the  ventral  (motor)  radic- 

lined  with  ependymal  cells,  outside  of  i  11        /-     \   ^i  j         11    ^         1     x 

which  lies  the  neuroglia.   X  135.  ul^r  cclls;  (3)  the  axoucs  and  collaterals  from 

the  numerous  cells  of  the  posterior  horn  that 
traverse  the  gray  matter  to  and  from  the  respective  columns  in  which  they 
course.     The  dendritic  processes  also  contribute  to  the  complex. 

The  W^hite  Matter. — Since  the  predominating  components  of  the 
white  substance  are  longitudinal  nerve-fibres  that  pass  for  a  longer  or  shorter 
distance  up  and  down  in  the  columns  of  the  cord,  in  transverse  sections  the 
field  between  the  gray  core  and  the  surface  is  made  up  chiefly  by  the  cross- 
sections  of  the  meduUated  nerve-fibres.  These  appear  as  small,  closely  set 
and  somewhat  compressed  rings  (Fig.  320),  enclosing  deeply  stained  dots, 
surrounded  by  faint  annular  striations.  The  dots  are  sections  of  the  axis- 
cylinders,  the  annular  striations  the  remains  of  the  framework  of  the  medul- 
lary coat,  and  the  rings  the  slight  condensations  of  the  neuroglia  that  take 
the  place  of  a  neurilemma,  which  is  wanting  on  the  fibres  of  the  cerebro- 
spinal axis.  The  indi\'idual  ner\e-fibres  vary  greatly  in  size,  even  within 
the  same  tract  large  and  small  ones  often  lying  side  by  side.  The  smallest 
may  be  less  than  5  //  in  diameter  and  the  largest  over  25  p..  In  general,  the 
thickest  fibres  are  found  in  the  anterior  and  in  the  lateral  part  of  the  posterior 
columns,  and  the  thinnest  ones  in  the  posterior  and  lateral  columns  in  the 
immediate  vicinity  of  the  gray  matter. 
18 


274 


NORMAL    HISTOLOGY. 


The  immediate  surface  of  the  white  matter,  beneath  the  investing  pia 
mater,  is  formed  by  a  tract  of  condensed  neuroglia,  the  subpial  layer,  that 
is  devoid  of  nervous  elements  and  constitutes  the  definite  outer  boundary  of 
the  cord.  This  zone  consists  of  a  dense  interlacement  of  circular,  longi- 
tudinal and  radial  neuroglia  fibrils,  along  with  many  glia  cells.  From  the 
deeper  surface  of  this  investment  numerous  bundles  of  fibrillce  penetrate 
between  the  nerve-fibres  and  become  lost  in  the  general  supporting  ground- 
work. At  certain  points  the  bundles  are  replaced  by  robust  septa  which 
imperfectly  separate  the  nerve-fibres  into  groups  or  tracts,  as  conspicuously 
exampled  by  the  paramedian  septum  subdividing  the  posterior  column.      The 


Fir..  320. — Part  of  periphery  of  cord,  showing-  subdivision  of  white  matter  by  ing:rowths  of  subpial  layer 

of  neuroglia.    X  230. 

blood-vessels  that  enter  the  nervous  substance  of  the  cord  from  the  pia  mater, 
accompani£d  by  connective  tissue,  are  surrounded  by  tubular  sheaths  of 
neuroglia,  and  the  same  is  true  of  the  bundles  of  root-fibres  of  the  spinal 
nerves.  With  the  exception  of  the  connective  tissue  that  enters  with  the 
blood-vessels,  the  amount  of  mesodermic  tissue  included  in  the  supporting- 
framework  of  the  cord  is  inconsiderable,  if,  indeed,  not  wanting. 

The  Fibre-Tracts. — Although  microscopical  examination  of  ordinary 
sections  of  the  cord  affords  slight  indication  of  a  subdivision  of  the  columns 
of  white  matter  into  areas  corresponding  with  definite  fibre-tracts,  yet  the 
combined  evidence  contributed  by  anatomical,  pathological,  embryological 
and  experimental  investigation  establishes  the  existence  of  a  number  of  such 
paths  of  conduction.      With  few  exceptions,  however,  they  are  without  sharp 


THE   SPINAL   CORD. 


?75 


boundaries  and  illy  defined,  adjoining  tracts  often  oxerlapping,  and  depend 
for  their  presence  upon  the  fact  that  nerve-tibres  haviny;  the  same  origin, 


Fig.  321. — Diagram  of  spinal  cord,  sliowing  position  of  chief  tracts  and  relations  of  their  component 
fibres  to  nerve-ceHs  ;  1-5,  posterior  root-fibres  entering  root-zone  (R.Z.)  and  Lissauer's  tract  (L.),  open 
circles  (o)  indicate  that  fibres  pass  up  and  down  ;  c,  c,  collaterals  from  long  ascending  tracts  (i,  2)  to 
anterior  root-cells;  3,  fibres  ending  around  cells  of  Clarke's  column;  6,  fibres  forming  direct  cerebellar 
tract ;  7,8,  fibres  forming  Gowers'  tract ;  9,  10,  fibres  from  lateral  and  direct  pyramidal  tract ;  11. 11,  it,  an- 
terior root-fibres  ;  12,  association  tracts;  13,  rubro-spinal ;  14,  17,  \estibulo-spinal ;  15,  spino-thalamic  ;  16, 
olivo-spinal  tracts. 


function  and  destination  usually 
tion  to  being  provided  with 
paths  of  conduction  necessary 
for  the  performance  of  its  func- 
tion as  a  centre  for  independent 
or  reflex  impulses  in  response 
to  external  stimuli,  the  cord 
contains  tracts  that  connect  it 
with  the  brain,  as  well  as  those 
that  bring  the  various  levels  of 
the  cord  itself  into  association. 
The  white  matter  contains, 
therefore,  three  classes  of  ner\-e- 
fibres:  (i)  those  entering  the 
cord  from  the  periphery  and 
other  parts  of  the  body;  (2) 
those  entering  it  from  the 
brain;  and  (3)  those  arising 
from  the  ner\-e-cells  situated 
within  the  cord  itself.  It  is 
evident  that  some  of  these  fibres 
constitute  pathways  for  the 
transmission  of  impulses  from 
lower  to  higher  levels  and  hence 
conduct  impulses  in  the  opposite 


proceed  along  a  similar  path.      In  addi- 


/;«i 

M^^ 


^Pt-^ 


Fig.  322. — Section  of  spinal  cord  at  level  of  second  cer- 
vical segment ;  formiitio  reticularis  fills  bay  between  pos- 
terior and  anterior  coniua  ;  substantia  gelatinosa  caps  apex 
of  posterior  cornu.  (Drawn  from  Weigert-Pal  preparation 
made  by  Professor  Spiller. )     X  6. 

form  ascending  tracts,  while  others,  which 
direction,  form  descending  tracts. 


276  NORMAL    HISTOLOGY. 

Based  on  the  collective  results  of  the  anatomical,  physiological  and 
developmental  methods  of  study,  it  is  possible  to  locate  and  trace  with  fair 
accuracy  a  number  of  fibre-tracts  in  the  cerebro-spinal  axis.  Since  they  are 
undert^oing  continual  augmentation  or  decrease,  their  actual  area  and  posi- 
tion are  subject  to  variation,  so  that  the  detailed  relations  in  one  region  of 
the  cord  differ  from  those  at  other  levels.  The  accompanying  schematic 
fto-ure  (Fig.  321),  therefore,  is  to  be  regarded  as  showing  only  the  general 
relations  of  the  most  important  paths  of  the  cord,  and  not  as  accurately 
representing  the  actual  form  and  size  of  the  fibre-tracts.  Further,  it  must 
be  appreciated  that  the  definite  limits  of  these  tracts  in  such  diagrammatic 
representations  seldom  exist  in  reality,  since  the  fibres  of  the  adjacent  paths 
in  most  cases  overlap,  or,  indeed,  extensively  intermingle,  so  that  the  fields 
marked  in  cross-sections  may  be  shared  by  fibres  belonging  to  differen 
systems. 

The  constitution  of  the  fibre-tracts  of  the  cerebro-spinal  axis  is  mani- 
festly beyond  the  province  of  these  pages.      For  such  information  the  reader 


"% 


Fig.  323.— Section  of  cord  through  lower  part  of  cervical  region.     /  6.    ( Preparation  by  Professor  Spiller.) 

is  referred  to  the  systematic  text-books  of  anatomy  and  the  special  works  on 
the  nervous  system.  A  few  considerations  of  importance,  however,  may 
here  find  appropriate  mention. 

All  afferent  or  sensory  impulses  entering  the  cord  do  so  by  way  of  the 
posterior  root-fibres.  The  latter  are  the  centrally  directed  processes  (axones) 
of  the  neurones  whose  cell-bodies  are  the  ganglion-cells  within  the  spinal 
ganglia  situated  on  the  dorsal  roots  of  the  spinal  nerves.  These  root-fibres 
convey  to  the  cord  the  various  impulses  collected  by  the  peripherally  directed 
processes  (the  sensory  nerves)  from  the  integument,  mucous  membranes, 
muscles,  tendons  and  joints  from  all  parts  of  the  body,  with  the  exception  of 
those  served  by  the  cranial  nerves.  Some  of  the  afferent  impulses  thus  con- 
ducted are  transformed  in  the  cerebrum  into  impressions  of  touch,  muscle- 
sense  and  temperature,  while  others  are  carried  to  the  cerebellum,  which 
responds  by  efficient  impulses  that  exercise  the  restraint  necessary  to  maintain 
coordination. 

On  entering  the  spinal  cord  along  the  postero-lateral  sulcus,  most  of 
the  dorsal  root-fibres  penetrate  the  fasciculus  cuneatus  (tract  of  Burdach) 


THE   SPINAL   CORD.  277 

and,  with  few  exceptions,  undergo  a  >-  or  K-  like  branching  into  ascend- 
ing and  descending  limbs,  which  assume  a  longitudinal  course  and  pass  up 
and  down  in  the  cord  for  a  variable  distance.  During  their  course  both 
limbs  give  off  collateral  branches  which  bend  sharply  inward  and  pass  hori- 
zontally into  the  gray  matter  to  end  chiefly  in  relation  with  the  cells  of  the 
posterior  horns,  from  which  cells  new  secondary  paths  arise.  The  main 
stem-fibres  of  the  descending  and  of  the  short  ascending  limbs  also  end  in 
the  manner  just  described.  In  addition  to  the  short  collaterals  destined  ^or 
the  cells  of  the  dorsal  horn,  others,  the  ventral  reflex  collaterals,  traverse 
the  posterior  horn  and  intermediate  gray  matter  to  end  in  arborizations 
around  the  ventral  radicular  cells,  and  thus  complete  important  reflex  arcs 
by  which  impulses  transmitted  through  the  dorsal  roots  directly  imj)ress  the 
motor  neurones.  With  possibly  the  exception  of  certain  fibres  said  to  pass 
directly  to  the  cerebellum,  all  the  sensory  root-fibres  end  around  neurones 
situated  either  within  the  gray  matter  of  the  spinal  cord  or  within  the  nuclei 
of  the  medulla.  Thence  the  impres- 
sions are  conveyed  by  axones  of  -^^^'-'.'■ 
these  secondary  neurones  to  higher  :^?A-^^4jif  ^'' '' 
centres,  to  be  taken  up  in  turn  by  ^.^^^^''^u^-^i^r.  V 
tertiary  neurones,  in  the  sequence  .■^^ 
of  the  chain  required  to  complete  /'^' 
the  path  for  the  conduction  and 
distribution  of  the  impulse.  /. 
The  long  ascending  poste-  !'l 
rior  tract  includes  the  dorsal  root-  ^  ^1 
fibres  that  pass  uninterruptedly  up-  '  ^^^M 
wards  within  the  posterior  column  "'  '  ^ 
(within  the  tracts  of  GoU  and  of 
Burdach)  as  far  as  the  nuclei  (graci-  "^  - 
lis  and  cuneatus)  of  the  medulla.  -  -^^ 
Many   other  root-fibres,    however,      ^            _    .•       c      j  .u       u    ■  ui     t  »u 

-■  '.  '        Fig.  324. — Section  of  cord  through  middle  of  thoracic 

ascend  for  only  a  short  distance  and  region.     X  6.     (Preparation  by  PrcfcssorSpiller.) 

then  bend  inwards  to  end  around 

the  cells  of  the  posterior  horn.  Among  the  most  important  of  such  fibres 
are  those  that  pass  to  the  neurones  of  Clarke's  column  (page  271),  around 
which  they  end  in  telodendria.  The  axones  of  these  neurones  continue  the 
path  for  the  impulses  received  from  the  dorsal  root-fibres  by  cutting  across 
the  grav  matter  and  lateral  column  to  the  periphery  where,  bending  brain- 
wards,  they  form  the  important  direct  cerebellar  tract  {fasciculus  cere- 
bello-spinalis  ) . 

Among  the  many  neurones  of  the  posterior  horn  around  which  the 
dorsal  root-fibres  end,  some  send  their  axones  into  the  lateral  columns  to 
form  the  ascending  tract  of  Gowers  {fasciculus  antero-lateralis  superfi- 
cialis),  which  occupies  the  periphery  of  the  cord  immediately  in  advance  of 
the  direct  cerebellar  tract.  Others  send  their  axones  into  the  antero-lateral 
column  of  the  opposite  side  and  ascend  as  the  spino-thalamic  tract,  a 
sensory  pathway  of  importance  but  diffuse. 

The  foregoing  tracts  are  all  concerned  in  transmitting,  directly  or 
indirectly,  the  impulses  brought  by  the  dorsal  root-fibres  to  higher  levels; 
they  are  all,  therefore,  ascending  paths.  In  order,  however,  that  the  spinal 
cord  with  its  long  series  of  motor  nerves  shall  be  brought  under  the  influence 
of  the  volitional  and  coordinating  impulses  arising  within  the  cerebrum  and 
cerebellum  respectively,   it   is  evident  that  descending  tracts  composed  of 


278  NORMAL    HISTOLOGY. 

efferent  fibres  exist.  Such  fibres,  connecting  the  motor  cells  of  the  cerebral 
cortex  with  the  motor  root-cells  of  the  spinal  nerves,  are  condensed  into  two 
chief  strands,  the  anterior  and  lateral  pyramidal  tracts.  The  latter 
{fasciculus  cerebro-spinalis  lateralis)  occupies  an  oval  area  between  the 
lateral  aspect  of  the  posterior  horn  and  the  direct  cerebellar  tract.  The 
component  fibres  are  the  axones  of  the  motor  cortical  neurones,  which  have 
descended  from  the  white  core  of  the  cerebrum,  through  the  ventral  part  of 
the  brain-stem  (cerebral  peduncle,  pons  and  medulla),  to  the  lower  part  of 
the  medulla  and  thence,  after  crossing  in  the  pyramidal  decjcssation  (page 
283),  into  the  lateral  column  of  the  cord.  On  reaching  the  level  correspond- 
ing to  the  cord-segment  to  be  influenced,  the  cerebro-spinal  fibres  bend 
inwards,  enter  the  gray  matter,  and  end  around  the  motor  root-cells  in 
arborizations.  The  fibres  of  the  anterior  or  direct  pyramidal  tract 
{fasciculus  cerebro-sphialis  anterior^  correspond  in  origin  and  course  with 
those  of  the  lateral  tract  ■  until  they  reach  the  lower  part  of  the  medulla, 
where,  instead  of  crossing  in  the  pyramidal  decussation,  they  continue  into 
the  anterior  column  and  descend  within  a  narrow  zone  along  the  anterior 
median  fissure  of  the  cord.  Although  not  sharing  the  decussation  in  the 
medulla,  practically  all  of  these  fibres  cross  somewhere,  by  way  of  the  anterior 

white  commissure,  and  pass  at  the 

g:^,%  appropriate  level    into   the  anterior 

~  ~^J^"^"'V3  cornu  of  the  opposite  side,  to  end  in 

fli£     ^  ^iJ^^L  arborizations  around    the   radicular 

/     "   t    'i  cells.      In  no  case  does  a  motor  im- 

Z''  "X  pulse  pass  directly  from  the  cerebral 

^-A  cortex   to    the    muscle    fibre,    since 

''\        always  at  least  two  links — the  corti- 
\  -      "  -  \       cal  and  the  spinal  neurone-^are  re- 

r  ^         f      quired  to  complete  the  chain. 

'i  The    existence    of  direct  paths 

'/       from  the  cerebellar  cells  to  those  of 

-  the  spinal  cord  is  uncertain,  the  im- 

\  V  pulses  from    the    cerebellum    being 

"—    — -  -^  -  ~  -.^.^:;f^^^  -^  usually  carried  by  the  axones  of  the 

Fig.  325.-section  of  cord  through  lower  part  of     Cerebellar  ncuroncs  to    cells  within 

'^mhar  legiou.  X  6.  (Preparalion  by  Professor  ^^g  braiu-Stem  (red,  vestibular  and 
Spiller.)  ,    .       ^         ' 

olivary  nuclei)  and  thence  by  secon- 
dary axones  (rubro-,  vestibulo-,  and  olivo-spinal  fibres)  through  the  antero- 
lateral columns  to  the  gray  matter  of  the  cord. 

Although  the  tracts  above  described  include  the  more  definite  and 
evident  of  the  paths  of  conduction,  a  glance  at  Fig.  321  shows  that  a  con- 
siderable part  of  the  anterior  and  lateral  columns  of  the  cord  is  still  un- 
accounted for.  Concerning  this  extensive  area,  to  which  the  name  antero- 
lateral ground-bundle  is  conveniently  applied,  much  uncertainty  exists; 
suflice  it  to  point  out,  that  within  its  complex  are  many  fibres  of  importance, 
some  of  which  connect  the  cord  with  distant  parts  of  the  brain,  while  others 
serve  as  links  binding  together  different  levels  of  the  cord  itself.  These  last, 
the  intersegmental  association  fibres,  are  for  the  most  part  short, 
passing  as  the  axones  of  the  posterior  horn  cells  into  the  adjacent  white 
matter  and,  after  a  limited  course,  bending  inwards  to  enter  once  more  the 
gray  matter  and  end  around  the  nerve-cells  at  some  different  level.  The 
shorter  association  tracts  lie  close  to  the  gray  matter,  while  the  longer  ones 
run  within  the  more  peripheral  part  of  the  antero-lateral  ground-bundle. 


THE    BRAIN. 


279 


The  blood-vessels  supplyinu:  the  spinal  cord,  derived  from  many 
sources,  form  an  arterial  network  within  the  pia  mater  that  accompanies  the 
nervous  cylinder  throui^hout  its  length.  The  gray  matter  recei\^es  its 
principal  supply  from  the  series  of  anterior  fissural  arteries,  over  two  hun- 
dred in  number,  that  pass  from  the  anterior  spinal  trunk  backwards  within 
the  anterior  median  fissure  to  its  bottom  and  there  di\ide  into  right  and  left 
branches,  which  traverse  the  ante- 
rior white  commissure  to  gain  the 
gray  matter  on  either  side  of  the 
central  canal.  These  vessels,  the 
Sitlco-viargiual  arteries,  di\'ide  into 
ascending  and  descending  branches 
that  provide  a  rich  capillary  net- 
work for  the  entire  gray  matter, 
with  the  exception  of  its  most  pe- 
ripheral zone.  The  latter,  together 
with  the  white  matter,  receives  its 
supply  from  the.  penetrating  aiieri- 
olcs  that  come  from  the  surrounding 
intrapial  trunks  and  enter  the  sub- 
jacent surface  of  the  cord.  Unpaired 
horizontal  twigs,  the.  posterior  sul- 
cal  arteries,  follow  the  posterior 
median  septum  at  different  levels 
for  some  distance,  but  before  reach- 
ing the  posterior  gray  commissure 
usually  break  up  into  terminal 
ramifications,  some  of  which  pass 
to  the  gray  matter  of  the  posterior 
horns.  Notwithstanding  the  capil- 
lary anastomoses  within  the  nervous  tissue,  each  artery  provides  the  sole 
available  supply  for  some  definite  territory;  they  are,  therefore,  "end- 
arteries,"  a  fact  which  explains  the  extensive  and  elaborate  system  of  vessels 
necessary  to  maintain  the  nutrition  of  the  cord.  The  plexiform  veins  w'ithin 
the  spinal  pia  are  formed  by  the  union  of  the  small  radicles,  that  collect  the 
blood  from  the  intraspinal  capillaries  and  emerge  at  the  surface  of  the  cord. 
Definite  lymphatics  within  the  spinal  cord  are  unknown. 

The  investing    membranes  or   meninges   of   the  spinal   cord  are 
described  in  connection  with  those  of  the  brain  (page  311). 


Fig.  326.— Transverse  section  of  injected  cord,  show- 
ing; vascular  supply  of  white  and  gray  matter;  a, 
anterior  spinal  giving  off  anterior  sulcal  {b)\  c,  ascend- 
ing branch  ;  d,  perforating  arteries  ;  e,  poslero-lateral, 
/,  parasulcal ;  g,  posterior  sulcal.     X  lo. 


THE   BRAIN. 

Before  entering  upon  the  description  of  the  histological  details  of  the 
more  important  parts  of  the  brain,  it  is  most  desirable  to  have  some  notion 
of  their  general  position  and  relations.  A  brief  survey  of  the  gross  anatomy 
of  the  human  brain,  therefore,  is  an  advantageous  introduction  to  the  study 
of  its  structure. 

Denuded  of  its  investing  membranes  and  the  attached  cranial  nerves, 
and  viewed  from  below  (Fig.  327),  the  brain,  or  encephalon,  is  seen  to  con- 
sist of  a  median  brain-stem,  that  inferiorly  is  directly  continuous  with  the 
spinal  cord,  through  the  foramen  magnum,  and  abo\'e  divides  into  two  diverg- 
ing arms  that  disappear  within  the  large  overhanging  mass  of  the  cerebrum. 
The  brain-stem  includes  three  divisions,  the  inferior  of  which,  the  medulla 


28o 


NORMAL   HISTOLOGY. 


oblongata,  is  the  uninterrupted  upward  prolongation  of  the  spinal  cord  and 
above  is  limited  by  the  projecting  lower  border  of  the  quadrilateral  mass  of 
the  next  division,  \\-\(t  pons  Varolii.  Beyond  the  upper  margin  of  the  pons, 
the  brain-stem  is  represented  by  a  third  division  that  ventrally  is  separated 
by  a  deep  recess  into  two  diverging  limbs,  the  cerebral  peduncles,  or  crura 
cerebri,  to  correspond  with  the  halves  or  hemispheres  of  the  cerebrum. 
Each  of  these  receives  one  of  the  crura  and  in  this  manner  is  connected  with 
the  lower  levels  of  the  cerebro-spinal  axis.  The  greater  part  of  the  medulla 
and  pons  is  covered  dorsally  by  the  cerebellum,  whose  large  lateral  expan- 
sions, or  hemispheres,  project  on  either  side  as  conspicuous  masses,  distin- 


Orbital  surface  of 
frontal  lobe 


Optic  commissure 

Optic  tract 

Cerebral  peduncle 

Interpeduncular  space 


Medulla 
Cerebellum 


plfactory  tract 

Stalk  of  pituitary  body 

Tuber  cinereum 
Mammillary  bodies 
Cerebral  i>eduijcle 
Temporal  lobe 
Pons 


-  Cerebellum 


Occipital  lobe 


Fig.  327. 


Spinal  cord 


-Human  brain,  viewed  from  below,  showing  relations  of  brain-stem  to  spinal  cord  and  cerebrum, 
as  well  as  more  prominent  details  of  brain. 


guished  by  the  closely  set  plications  and  intervening  fissures  that  mark  their 
surface.  Of  the  five  component  parts  of  the  brain — medulla,  pons,  cerebral 
peduncles,  cerebrum,  and  cerebellum — the  last  two  are  coated  with  the  cor- 
tical gray  matter,  in  which,  broadly  speaking,  are  situated  the  neurones  that 
constitute  the  end-stations  for  the  sensory  impulses  conveyed  by  the  various 
corticipetal  paths  and  the  centres  controlling  the  lower-lying  nuclei  of  the 
motor  nerves.  The  brain-stem,  on  the  other  hand,  whilst  containing  numer- 
ous stations  for  the  reception  and  distribution  of  sensory  impulses,  is  pri- 
marily the  great  pathway  by  which  the  cerebrum  and  the  cerebellum  are 
connected  with  each  other  and  with  the  spinal  cord. 

Viewed  in  a  mesial  sagittal  section  (F'ig.  328),  each  of  these  divisions  is 
seen  to  be  related  to  some  part  of  the  system  of  communicating  spaces  that, 
as  the  lateral  and  third  ventricles,  the  aqueduct  of  Sylvius  and  the  fourth 
ventricle,  extend  from  the  cerebral  hemispheres  above,  through  the  brain- 


THE   BRAIN. 


281 


stem  and  beneath  the  cerebeUum,  to  the  central  canal  of  the  spinal  cord 
below.  Both  the  roof  and  the  floor  of  the  irregular  third  ventricle  are 
thin,  while  its  lateral  walls  are  formed  by  two  robust  masses,  \.\\q  optic  thalanu, 
the  mesial  surface  of  one  of  which  forms  the  background  of  the  space  when 
Aiewed  in  sagittal  section.  The  ?-oof  of  the  ventricle  is  very  thin  and  consists 
of  the  delicate  layer  of  cpcndynia,  as  the  immediate  lining  of  the  ventricular 
spaces  is  designated,  supported  by  the  closely  adherent  fold  of  pia  mater 
which  in  this  situation  pushes  before  it  the  neural  wall  and  contains  within 
its  lateral  border  a  thickened  fringe  of  blood-vessels,  the  choroid  plexus. 
The  two  structures,  the  ependyma  and  the  pia  mater,  together  constitute 
the  membranous  velum  intcrpositKm  that  forms  the  roof  of  the  ventricle. 
Behind,  just  over  the  upper  end  of  the  Sylvian  aqueduct,   lies  the  cone- 

Corpus  callosum 
Septum  lucidum 


Lateral  ^valI  of  third 
ventricle  (optic 
thalamus) 


Anterior  commissure 
Foramen  ot  M 
Lamina  cinerea 

Optic  commissure' 

Hituitar>'  boci 


Floor  of  third  ventricl 
Mammillary  bod 
Aqueduct  of  Sylvius 
Puns 

J-ourth  \eiitrii.l 


Cerebral  peduncle 


Ronf  of  Sylvian 
aqueduct 


Occipital  lobe 
•Superior  medullary  velu 

White  core  of  cerehellun 
Inferior  medullary  velum 


Fig.  32S. — Human  brain  seen  in  mesial  sagittal  section,  showing  relations  of  brain-stem,  cerebrum  and 

cerebellum  and  the  ventricles. 


shaped  pineal  body.  The  floor  of  the  third  ventricle  is  also,  for  the  most 
part,  thin  and  irregular.  It  corresponds  to  the  median  part  of  the  lozenge- 
shaped  area,  the  interpeduncular  space,  bounded  behind  by  the  diverging 
cerebral  peduncles  and  in  front  by  the  optic  chiasm  and  optic  tracts.  Passing 
forwards  from  the  deep  recess  between  the  cerebral  peduncles,  the  paired 
corpora  mammillaria,  the  tuber  cinereum  and  the  stalk  of  \.\\q  pituitary  body 
occupy  the  interpeduncular  space. 

The  Sylvian  aqueduct,  the  narrow  canal  connecting  the  third  and  fourth 
\entricles,  is  surrounded  ^'entrally  and  laterally  by  the  dorsal  part  or  tegmen- 
tum of  the  cerebral  peduncles.  Above  it  lies  a  plate  of  some  thickness, 
whose  free  dorsal  surface  is  modelled  bv  two  pairs  of  rounded  elevations,  the 
corpora  quadrigemina.  "W^o.  fourth  ventricle  in  sagittal  section  appears  as  a 
triangular  space,  the  anterior  or  basal  wall  being  the  dorsal  surface  of  the 
pons  and  medulla  and  the  posteriorly  directed  apex  lying  beneath  the  cere- 
bellum. When  viewed  from  behind,  the  ventricle  is  rhomboidal  in  outline, 
the  lateral  boundaries  above  being  the  superior  cerebellar  peduncles.,   that 


282 


NORMAL   HISTOLOGY. 


di\-ergingly  descend  from  the  sides  of  the  corpora  quadrigemina  to  the  cere- 
bellum ;  similar  arms,  the  inferior  cerebellar  peduncles,  also  known  as  the 
restiform  bodies,  convergingly  descend  from  the  cerebellar  hemispheres  to 
the  posterior  columns  of  the  medulla  and  forjxi  the  lower  lateral  boundaries 
of  the  fourth  ventricle.  Along  the  floor  of  the  fourth  ventricle  and  of  the 
Sylvian  aqueduct  lies  an  important  sheet  of  gray  matter,  continuous  with 
that  surrounding  the  central  canal  of  the  spinal  cord,  while  within  the  white 
matter  of  each  cerebral  hemisphere  are  embedded  two  large  gray  masses, 
the  corpus  sb^iatiun  and  the  optic  thalamns,  often  together  termed  the  basal 
gajiglia.  The  optic  thalamus  is  of  especial  importance  as  the  station  towards 
which,  in  a  general  way,  all  afferent  (sensory)  impulses  destined  for  the  brain 
converge. 

THE   MEDULLA   OBLONGATA. 

The  medulla  oblongata,  sometimes  called  the  bulb  and  usually  desig- 
nated by  the  convenient  but  indefinite  name  "medulla,"  is  the  lowest  seg- 
ment of  the  brain-stem  and  the  direct  upward  prolongation  of  the  spinal 

cord.  It  is  regarded  as  beginning  be- 
low at  the  lower  border  of  the  foramen 
magnum  and  extends  upwards  to  the 
lower  margin  of  the  pons,  a  distance  of 
about  2. 5  cm.  Its  general  form  is  taper- 
ing, increasing  in  breadth  from  the 
transverse  diameter  of  the  cord  (lo 
mm. )  below,  to  about  twice  as  much 
(i8  mm. )  above,  and  in  the  antero-pos- 
terior  dimension  from  8  to  15  mm. 

In  many  respects  the  medulla  ap- 
pears to  be  the  direct  continuation  of 
the  spinal  coi'd.  Thus,  it  is  divided 
superficially  into  symmetrical  halves  by 
the  anterior  and  posterior  median  fis- 
sures; each  half  is  subdivided  by  a 
ventro-lateral  and  a  dorso-lateral  line  of 
nerve-roots  into  tracts  that  seemingly 
are  continuations  of  the  columns  of  the 
cord.  This  correspondence,  however, 
is  onlv  superficial,  cross-sections  of  the  medulla  revealing  the  presence  of 
considerable  masses  of  gray  matter  and  important  tracts  of  nerve-fibres  not 
represented  in  the  cord,  as  well  as  the  rearrangement,  modificatior  or  disap- 
pearance of  spinal  components  prolonged  into  the  bulb. 

As  just  stated,  each  half  of  the  medulla  is  subdivided  into  three  longi- 
tudinal tracts,  the  ante^nor^  lateral  and  posterior  areas,  by  two  grooves 
situated  at  some  distance  from  the  mid-line.  Of  these,  the  antero- lateral 
fiLrrow  marks  the  emergence  of  the  root-fibres  of  the  hypoglossal  nerve, 
which  are  entirely  motor  and  correspond  to  the  ventral  roots  of  the  spinal 
nerx-es,  with  which  they  are  in  series.  The  other  groove,  the  postero- 
lateral furrow,  marks  the  attachment  of  the  fibres  of  the  ninth,  tenth  and 
bulbar  part  of  the  eleventh  cranial  nerves.  These  fibres  are  both  afferent  and 
efferent  and  do  not  correspond  to  the  posterior  roots  of  the  spinal  nerves. 

Seen  in  transverse  sections,  the  medulla,  even  at  its  lower  end,  presents 
new  features,  and  towards  its  upper  limit  varies  so  greatly  from  the  cord 
that  only  slight  resemblance  to  the   latter  is  retained.      The  characteristic 


Spinal 
cord 


Fig.  329. — Ventral  aspect  of  brain-stem  ;  the 
lines  lettered  on  rijjht  side  indicate  the  levels  of 
succeeding  cross-sections. 


THE  MEDULLA  OBLONGATA 


283 


features  displayed  by  sections  of  the  medulla  at  different  levels  depend  upon 
the  chang-es  induced  chiefly  by  four  factors:  (i)  the  decussation  of  the 
pyramidal  tracts,  (2 )  the  appearance  of  the  dorsal  nuclei,  (3)  the  production 
of  the  formatio  reticularis,  and  (4)  the  opening  out  of  the  fourth  ventricle. 

The  decussation  of  the  pyramidal  tracts,  assuming  for  conve- 
nience that  the  fibres  pass  from  below  upwards,  produces  conspicuous  changes 
when  followed  in  consecutive  sections  from  the  spino-bulbar  junction  upwards. 
The  decussation  first  appears  as  strands  of  fibres  that  pass  from  the  field  of 
the  lateral  pyramidal  tract,  in  the  lateral  column  of  the  cord,  obliquely 
through  the  adjacent  anterior  horn  of  gray  matter  and  across  the  bottom  of 
the  anterior  median  fissure  to  gain  the  opposite  anterior  area  of  the  medulla. 
At  a  slightly  higher  level  (Eig.  330 j,  where  the  decussation  is  fully  estab- 


Nucleus  j-nc  ilis 


Pyramidal  decussation 


Fig.  330. — Section  across  medulla  at  level  A,  Fis:.  329  ;  fibres  of  pyramidal  decussation  almost  fill  anterior 
median  fissure;  posterior  horns  are  displaced  laterally  by  increased  posterior  columns.  K  s'A.  (Prepa- 
ration by  Professor  Spiller.) 

lished,  the  large  strands  of  obliquely  sectioned  fibres  are  seen  cutting 
through  the  gray  matter,  partly  filling  the  median  fissure,  and  collecting  on 
either  side  of  the  latter  as  the  large  ventral  bundles  which  thence  upwards 
constitute  the  prominent  pyramidal  fields.  In  consequence  of  the  greater 
space  required  by  the  pvramids,  from  80-90  per  cent,  of  the  fibres  having 
crossed,  the  isolated  anterior  horns  of  the  gray  matter  (cut  oft"  by  the 
crossing  strands)  and  the  adjacent  anterior  ground-bundles  are  displaced 
laterally.  Higher,  the  ground-bundle  assumes  a  position  behind  the  pyramid 
and  eventually  becomes  continuous  with  the  posterior  longitudinal  fasciculus. 
The  detached  anterior  cornu  is  pushed  outwards  and  backwards  and  gradually 
becomes  broken  up  by  and  interspersed  among  the  fibres  of  the  formatio 
reticularis. 

The  posterior  nuclei  include  two  new  masses  of  gray  matter,  the 
micleus  gracilis  and  the  nucleus  cuneatus  (Eig.  331),  into  which  the  long 
posterior  tracts  of  the  cord  (Goll  and  Burdach)  are  prolonged.  The 
gracile  nucleus,  the  first  encountered  in  passing  upwards,  begins  on  a  level 
with  the  pyramidal  decussation  and  rapidly  increases  in  bulk  until  it  invades 
the  entire  funiculus  gracilis.  The  superficial  stratum  of  spinal  fibres,  pro- 
longed from  GoU's  tract,  gradually  diminishes  as  more  and  more  of  its  com- 
ponents end  in  arborizations  around  the  cells  of  the  gracile  nucleus,  until, 
finally,  all  are  interrupted.      These  neurones  are  multipolar  and  of  \'arying 


284 


NORMAL    HISTOLOGY. 


size;  some  are  small,  with  dendrites  ramifying  close  to  the  cell-body,  and 
others,  both  large  and  small,  with  widely  spreading  branched  processes. 
Meanwhile  the  cuneate  nucleus  appears  within  the  funiculus  cuneatus  as 
a  club-shaped  mass  of  gray  matter,  which  soon  becomes  a  prominent  mottled 
area.  This  nucleus,  also  composed  of  neurones  with  contracted  or  with  widely 
spreading  dendrites,  extends  to  a  higher  level  than  the  nucleus  gracilis. 

Owing  to  the  increased  bulk  of  the  fasciculi  of  the  posterior  area  occa- 
sioned by  the  appearance  and  expansion  of  the  gracile  and  cuneate  nuclei, 
the  dorsal  horns  of  the  gray  matter  are  displaced  laterally  and  ventrally,  so 
that  they  come  to  lie  on  a  level  with  the  central  canal.  Moreover,  the 
posterior  cornua  themselves,  particularly  the  capping  substantia  gelatinosa, 


Fig.  331. — Section  across  medulla  a  few  millimeters  above  level  A,  Fig^.  329,  showing-  increased  pos- 
terior nuclei  and  substantia  gelatinosa,  sensory  decussation  and  pyramidal  tracts,  i,  funiculus  gracilis  ; 
2,  funiculus  cuneatus;  3.  spinal  root  of  fifth  nerve;  4,  substantia  gelatinosa  ;  5,  accessory  olivary  nucleus; 
6,  displaced  antero-lateral  ground-bundle;  7,  superficial  arcuate  fibres;  8,  pyramidal  tracts;  9,  sensory 
(fillet)  decussation  ;  10.  fibres  of  twelfth  nerve  :  11,  deep  arcuate  fibres;  12,  central  gray  matter,  surround- 
ing canal ;  13,  nucleus  cuneatus;  14,  nucleus  gracilis.     X  5/i-     (Preparation  by  Professor  Spiller.) 

gain  materially  in  bulk  and  now  appear  as  two  club-shaped  masses  of  gray 
matter  (Fig.  331)  that  cause  the  dorso-lateral  projections,  the  tubercula 
Rolandi,  seen  on  the  surface.  Beneath  these  elevations  and  closely  over- 
lying the  areas  of  the  substantia  gelatinosa,  crescentic  tracts  of  longitudinally 
coursing  ner\'e-fibres  mark  the  position,  one  on  each  side,  of  the  spinal  roots 
of  the  trigeminal  nerves. 

The  chief  purpose  of  the  gracile  and  cuneate  nuclei  being  the  reception 
of  the  long  afferent  tracts  continued  from  the  cord  and  the  distribution  of 
impulses  so  received  to  the  cerebellum  and  to  the  higher  centres,  new  paths 
arise  within  these  nuclei.  About  the  level  of  the  upper  limit  of  the  pyramidal 
decussation,  axones  of  their  neurones  emerge  from  the  gracile  and  cuneate 
nuclei  as  the  deep  arcuate  Jibres,  sweep  forwards  and  inwards  in  bold  curves 
and  cross  the  mid-line  to  the  opposite  side  of  the  medulla.  Most  of  them 
then  turn  sharply  upwards  and  form  the  beginning  of  the  important  sensory 
pathway  known  as  the  median  fillet  {lemniscics  medialis).  The  lowest 
fibres  that  cross  in  this  manner  constitute  a  fairly  well  defined  strand,  the 
sensory  decussation  or  decussation  of  the  fillet.     The  crossing  does  not  cease 


THE  mp:dulla  oblonc;ata. 


285 


with  this  decussation,  lor,  on  the  contrary,  it  is  only  the  beginninj^-  of  an 
extended  series  of  afferent  arcuate  hbres  that  pass  across  the  niid-Hne  at 
various  levels  throughout  the  brain-stcni.  Since  many  longitudinal  fibres 
are  encountered  by  those  sweeping  from  side  to  side,  an  interweaving  of 
A^ertical  and  horizontal  fibres  takes  place,  which  results  in  the  characteristic 
formatio  reticularis  that  constitutes  a  large  area  within  the  medulla  (Fig. 
333),  as  well  as  within  the  dorsal  or  tegmental  portions  of  the  pons  and 
cerebral  peduncles. 

The  olivary  nuclei  include,  in  each  half  of  the  medulla,  three  masses 
of  gray  matter — the  inferior  olivary  nucleus  and  the  two  accessory  olivary 
nuclei.  The  inferior  olivary  nucleus  is  a  corrugated  sac-like  lamina  of  gray 
matter  which  underlies  and  causes  the  conspicuous  o\'al  elevation,  the  oliva, 

^- — '^-^^^ga^-  Cerebello-olivan.'  strands 


*€:, 

^f:: 


Gray 
matter 


Core  of 

wliite 
matter 


Fk 


-Section  of  inferior  olivary  nucleus,  sliovvins;  plicated  sheet  of  Kray  matter  traversed  by  strands 
of  nerve-fibres.     X  too. 


that  occupies  the  upper  half  of  the  medulla  at  the  outer  side  of  the  pyramid. 
In  favorable  cross-sections,  the  nucleus  appears  as  a  striking  sinuous  C-like 
figure  (Fig.  333),  the  mouth  of  the  sac  or  hilum  looking  mesially.  The 
greatest  length  of  the  nucleus  is  from  12-15  "'"''''•  '^'''<^  the  transverse  diameter 
about  half  as  much.  The  plicated  lamina  of  gray  matter,  from  .2-.  3  mm. 
thick,  contains  numerous  small  rounded  neurones,  from  18-26 //.in  diameter, 
each  provided  with  from  three  to  five  branched  dendrites  and  an  a.xone, 
embedded  within  a  complex  of  axis-cylinders  and  neuroglia.  The  interior 
of  the  gray  sac  is  filled  with  white  matter,  consisting  of  nerve-fibres,  that, 
for  the  most  part,  stream  through  the  hilum  and  constitute  the  olivary 
peduncle.  The  accessory  olivary  nuclei  are  two  irregular  plates  of  gray 
matter  that  lie  median  and  dorsal  to  the  chief  oli\'e.  The  median  nucletis, 
lo-ii  mm.  long,  is  sagittally  placed  between  the  tract  of  the  fillet  and  the 
root-fibres  of  the  hypoglossal  nerve.  The  dorsal  nucleus  is  less  extensive 
than  the  median  and  lies  close  to  and  behind  the  hilum  of  the  inferior  olive. 
In  structure  the  accessorv  nuclei  resemble  the  erav  matter  of  the  chief  one. 


286 


NORMAL   HISTOLOGY. 


The  central  canal  and  surrounding  gray  matter  are  gradually  dis- 
placed dorsally  within  the  closed  lower  half  of  the  medulla  in  consequence 
of  the  increasing  space  required  by  the  pyramids,  the  fillet-tract  and  the 
posterior  longitudinal  fasciculi — tracts  that  lie  close  to  the  median  raphe  and 
enlarge  as  they  are  followed  upwards.  Where  the  central  canal  opens  out 
into  the  fourth  ventricle,  the  surrounding  gray  matter  is  correspondingly 
spread  apart  and  becomes  the  Hning  of  the  ventricular  floor  (Fig.  333). 
Within  this  gray  sheet  and  near  the  mid-line,  on  each  side,  lies  the  group  of 
large  richly  branched  nerve-cells  (40-70  p.)  constituting  the  hypoglossal 
nucleus^  from  which  arise  the  fibres  of  the  twelfth  cranial  nerve.  These 
strands  take  a  direct  ventro-lateral  course  through  the  medulla  and  emerge 
on  the  anterior  surface,  in  the  groove  between  the  pyramid  and  the  olivary 
eminence  or  olive.      Slightly  lateral,  and  to  the  outer  side  of  the  hypoglossal 


Fig.  333.— Section  across  medulla  at  level  B,  Figf.  329;  i,  pyramidal  tracts  ;  2,  inferior  olivary  nucleus  ; 
3,  median  fillet  in  ventral  area  ;  4,  formatio  reticularis  alba  ;  5,  formatio  reticularis  grisea  ;  6,  7,  vestibular 
nucleus  and  root;  8,  restiform  body;  9,  torn  ventricular  i-oof ;  10,  posterior  longitudinal  bundle;  11, 
nucleus  of  XII  and  (12)  of  X  nerve  ;  13,  fasciculus  solitarius ;  14,  dorsal  area  ;  15,  fibres  of  X  nerve;  16, 
lateral  area;  17,  fibres  of  XII  nerve.      -(5.     (Preparation  by  Professor  Spiller.) 

nucleus,  another  group  of  cells  marks  the  position  of  the  dorsal  vago-accessory 
nucleus.  These  neurones  are  of  small  size  (30-40  /^  and  irregularly  stellate 
or  fusiform,  and  give  origin  to  part  of  the  motor  fibres  of  the  tenth  and 
bulbar  portion  of  the  eleventh  cranial  nerves.  The  root-fibres  of  the  vagus 
traverse  the  medulla  obliquely  and  meet  the  surface  in  the  dorso-lateral 
groove  marking  the  junction  of  the  posterior  and  lateral  divisions  of  the 
medulla. 

In  this  way,  the  diverging  fibres  of  the  tenth  and  twelfth  nerves  sub- 
divide each  half  of  the  medulla,  as  seen  in  transverse  sections,  into  three 
triangular  areas — the  dorsal,  the  lateral,  and  the  ventral. 

The  dorsal  area,  between  the  dorsal  surface  of  the  medulla  and  the 
vagus-fibres,  contains  a  number  of  important  fibre-tracts:  (i)  The  restiforui 
body,  or  inferior  Cerebellar  peduncle,  appears  as  a  large  crescentic  tract  of 
transversely  cut  fibres  that  occupies  the  greater  part  of  the  periphery.      (2) 


THE   MEDULLA    OBLONGATA. 


287 


The  descc7iding  root  of  the  vestibular  nerve  is  seen  as  a  field  of  loosely  grouped 
bundles  of  cross-sectioned  nerve-fibres  to  the  inner  side  of  the  restiform  body. 
(3)  The  fasciculus  solitarius  shows  as  a  conspicuous  transversely  cut  bundle 
that  lies  dorso-mesially  to  the  vestibular  root.  (4)  The  descendijig  root  of 
the  trigeminal  nerve  is  readily  identified  as  a  superficial  crescentic  field 
enclosing  on  its  mesial  aspect  the  substantia  geiatinosa. 

The  lateral  area,  between  the  diverging  vagal  and  hypoglossal  root- 
fibres,  is  occupied,  in  addition  to  (  i  )  the  inferior  olivary  and  (2)  the  dorsal 
accessory  7iucleus,  chiefly  by  the  feltwork  of  fibres  producing  the  formatio 
reticularis.  In  contrast  to  that  within  the  ventral  area,  the  reticulum  of  the 
lateral  area  contains  considerable  diffuse  gray  matter  between  its  fibres  and, 
hence,  is  known  as  (3)  th.e  formatio  reticularis  grisea.  Accessions  to  the 
irregularly  distributed  nerve-cells,  for  the  most  part  large  and  stellate  or 
fusiform,  occur  as  two  more  definite  collections;  one  of  these,  (4)  the  nucleus 


Nerve-cell 


Longitudinal.   '■'"^ 
fibres 


Fig.  334. — Portion  of  formatio  reticularis  grisea  of  medulla.     X  130. 


anibiguus,  consists  of  an  inconspicuous  group  of  large  cells  lying  about  the 
middle  of  the  gray  reticular  substance;  it  is  important  as  the  ventral  motor 
nucleus  giving  origin  to  at  least  a  part  of  the  motor  fibres  of  the  ninth  and  tenth 
nerves.  The  other  collection,  (5)  the  nucleiis  lateralis,  includes  an  uncertain 
aggregation  of  medium-sized  cells  situated  near  the  periphery,  ventral  to  the 
trigeminal  root.  A  separate  group  of  somewhat  larger  neurones,  near  the 
ventral  border  of  the  trigeminus,  is  the  nucleus  lateralis  dorsalis.  \\\  a 
general  way,  these  nuclei  (ambiguus  and  lateralis)  of  the  substantia  grisea 
may  be  regarded  as  homologues  of  the  lateral  horn  cells  of  the  cord,  just  as 
those  of  the  hypoglossal  nucleus  resemble  the  anterior  root-cells  of  the 
spinal  nerves. 

The  ventral  area,  between  the  mid-line  and  the  hypoglossal  root-fibres, 
is  occupied  ventrally  by  (  i )  the  pyramidal  tract,  which  appropriates  the 
entire  width  of  the  field  with  the  exception  of  a  very  narrow  peripheral  zone 
that  intervenes  between  the  pyramid  and  the  surface  along  the  median  fissure 
and  the  ventral  aspect  of  the  medulla.  This  zone  is  traversed  by  (2)  the 
anterior  superficial  arcuate  fibres,  among  which  lies  an  irregular  column  of 
nerve-cells,  (3 )  the  arcuate  nucleus.  Dorsal  to  the  pyramidal  tract  and  next 
the  mid-line,  lies  (4 )  the  compact  tract  of  the  mediaii  fillet,  composed  of 
longitudinal  strands  that  are  the  upward  continuations  of  the  deep  arcuate 
fibres  that  at  lower  levels  have  bent  brainwards,  after  crossing  the  mid-line. 


288  NORMAL   HISTOLOGY. 

The  fillet-tracts  are  also  known  as  the  interolivary  straticm,  since  they  form 
a  compact  and  compressed  field  between  the  inferior  olivary  nuclei.  Between 
the  fillet- tract  and  the  hypoglossal  fibres  is  (5)  the  mesial  accessory  nucleus. 
(6)  The.  posterior  longitudinal  fasciciilus  appears  in  cross-section  as  a  com- 
pact strand,  next  the  raphe  and  immediately  beneath  the  gray  matter  of  the 
floor  of  the  fourth  ventricle.  The  remaining  space  of  the  ventral  field, 
between  the  pyramid  and  the  ventricular  gray  matter,  is  occupied  by  (7)  the 


^-' 


■'CS?^^ 


^^ 


1 1/'    """^Wt?^ 

Nerve-  Longitudinal    Transverse 
cell  fibres  fibres 

Median  raphe 

Fig.  335. — Portion  of  formatio  reticularis  alba  of  medulla.     X  130. 

formatio  reticularis  alba,  so  designated,  in  distinction  to  the  formatio  grisea, 
on  account  of  the  meagre  amount  of  its  gray  matter  and  small  number  of  its 
nerve-cells,  since,  with  the  exception  of  in  the  immediate  vicinity  of  the  mid- 
line, where  the  miclezis  raphe  is  found,  these  are  almost  wanting. 

Although  at  higher  levels  additional  and  important  masses  of  gray 
matter  appear,  especially  those  related  to  the  auditory  nerve,  for  the  pur- 
pose of  these  pages  the  foregoing  general  description  of  the  medulla,  as 
seen  in  typical  section,  must  suffice. 

THE   PONS   VAROLH. 

Viewed  from  in  front,  the  pons  appears  as  a  quadrilateral  prominence, 
from  25—28  mm.  long,  interposed  between  the  medulla  below,  the  cerebral 
peduncles  above,  and  the  cerebellar  hemispheres  at  the  sides.  Its  lower 
and  upper  limits  are  well  defined,  but  at  the  sides  the  narrowed  mass  of  the 
pons  is  directly  continued,  downwards  and  backwards,  into  the  cerebellum  as 
middle  cerebellar  peduncles.  The  free  portion  of  the  dorsal  surface  of  the 
pons  forms  the  upper  half  of  the  floor  of  the  fourth  ventricle  and  is,  there- 
fore, not  visible  until  the  roof  of  that  cavity  is  removed.  Above  the  middle 
peduncles,  the  sides  of  the  pons  are  blended  with  the  overlying  superior 
cerebellar  peduncles,  which,  with  the  membranous  superior  medullary 
velum,  complete  the  ring  of  tissue  surrounding  the  narrow  upper  end  of  the 
fourth  ventricle. 

In  transverse  section  (Fig.  336),  the  pons  is  seen  to  include  two  clearly 
defined  areas,   the  ventral   and  the   dorsal.      The   ventral    part,   or  pars 


THE   PONS   VAROLII. 


289 


basalis,  is  largely  occupied  by  the  bulky  pyramidal  tracts,  which  are  now 
excluded  from  the  surface  by  a  conspicuous  layer  of  superficial  transverse 
fibres,  the  stratum  siiperjiciale  pontis,  that  laterally  sweeps  backwards  into 


Abducent  fibres 

Superior  ccrehellar  _ 
peduncle 
Facia]  fibres . 

Substantia  gelatinosa  - — 
Spinal  root  of  V  • — 
Facial  nucleus  . 


Suoerior  cerebellar  peduncle 
Nucleus  Post,  long         Nucleus 

of  \  I  fasciculus  ot  VI 


,-x 


,' Facial  fibers 
,  Vestibular  fibres 


■Spinal  root  of  V 
Olivary  peduncle 


buperior  olive 


Pyramidal  tracts 


Fig.  336. — Section  across  pons  at  level  C,  Fig.  329,  showing  subdivision  into  ventral  and  dorsal  areas. 
X  3-     (Preparation  by  Professor  Spiller.) 

the  cerebellar  peduncles.  The  pyramids,  however,  no  longer  appear  as 
compact  fields,  as  in  the  medulla,  but  are  broken  up  into  smaller  bundles  by 
the  transverse  strands  of    the  ponto-cerebcllar  fih'cs.     This  subdivision  is 


# 


'-f^>'.. 


'm 


Fig.  337. — Portion  of  ventral  area  of  pons,  showing  cells  of  pontile  nucleus.     X  300. 

more  marked  at  higher  levels  of  the  pons,  where  the  interweaving  of  the 
longitudinal  and  transverse  bundles  produces  a  coarse  feltwork,  the  stratum 
complexu7n.     At  the  upper  border  of  the  pons,   the  scattered  pyramidal 
19 


290 


NORMAL   HISTOLOGY. 


bundles  become  once  more  collected  into  two  compact  strands,  which  are 
continued  into  the  crusta  of  the  cerebral  peduncles.  The  dorsal  limit  of  the 
ventral  field  is  occupied  by  the  straUivi  profiindiim  pontis,  a  well  marked 
layer  of  deep  tranverse  fibres.  A  considerable  amount  of  gray  matter,  col- 
lectively known  as  the  pontile  nitcleiis,  is  distributed  within  the  interstices 
between  the  bundles  of  nerve-fibres.  The  numerous  cells  of  this  nucleus 
(Fig.  337),  generally  small  in  size  and  stellate  in  form,  are  related  to  the 
ponto-cerebellar  fibres  of  the  same  and  of  the  opposite  side,  many  being- 
stations  of  interruption  in  the  cortico-cerebellar  paths. 

The  dorsal  or  tegmental  part  of  the  pons  resembles  in  its  general 
structure  to   a   considerable   extent    the    formatio   reticularis    grisea    of   the 


-^""■S^^^^^^--^  '  f^J^     \ 


Fig.  338.— Section  across  pons  at  level  D,  Fig.  329;  i,  pyramidal  tracts;  2,  transverse  fibres  ;  3,  decus- 
sation and  (4)  arm  of  superior  cerebellar  peduncle ;  5,  nucleus  and  (6)  descending  root  of  V  nerve:  7, 
gray  matter  surrounding  (8)  Sylvian  aqueduct  ;  9, 10,  11,  parts  of  IV  nerve  and  its  decussation  ;  12,  lateral 
fillet;  13,  posterior  longitudinal  fasciculus  ;  14,  tegmental  area  ;  15,  mesial  fillet.  X  3-  (Preparation  by 
Professor  Spiller. ) 

medulla,  consisting  for  the  most  part  of  a  reticulum  of  transverse  and  longi- 
tudinal fibres,  interspersed  with  nerve-cells,  on  each  side  of  the  median 
raphe.  The  appearance  of  certain  masses  of  gray  matter  and  of  nerve- 
fibres,  together  with  changes  in  the  position  of  the  fillet,  produces  details 
that  vary  with  the  level  of  the  section.  When  this  passes  just  above  the 
lower  margin  of  the  pons  (Fig.  336),  two  diverging  and  obliquely  cut 
strands  of  fibres  mark  the  root-fibres  of  the  sixth  and  seventh  cranial  nerves 
and  divide  the  tegmental  region,  on  each  side,  into  three  areas.  The 
middle  area,  between  the  abducent  fibres  mesially  and  the  facial  ones 
laterally,  contains  three  important  collections  of  ner\'e-cells.  One  of  these, 
the  micleiis  of  the  sixth  nerve,  lies  close  to  the  floor  of  the  ventricle  and 
near  the  mid-line.  The  axones  of  these  neurones,  the  root-fibres  of  the 
abducent  (sixth  j  nerve,  take  an  oblique  ventral  .path  and  cut  through  not 


THE   CEREBRAL    PEDUNCLES.  291 

only  the  tegmental  but  also  the  \entral  part  of  the  pons  to  gain  its  lower 
border,  along  which  they  emerge.  Another  nucleus  of  the  middle  area,  the 
superior  olivary  nucleus,  lies  near  the  ventral  limit  of  the  tegmental  area, 
partly  lodged  within  an  indentation  on  the  dorsal  aspect  of  the  conspicuous 
tract  of  transverse  fibres,  the  corpus  irapezoidcs.  This  nucleus,  often  called 
the  superior  olive,  is  an  irregular  spherical  collection  of  neurones  interposed 
in  the  path  connecting  the  auditory  nuclei  with  the  cerebral  cortex  and 
cl()sely  related  with  the  tract  of  the  lateral  fillet.  A  small  collection  of 
nerve-cells  between  the  fibres  of  the  trapezoidal  tract  constitutes  the  nucleus 
trapczoides.  The  facial  nucleus  is  a  broken  mass  of  gray  matter  that 
includes  several  groups  of  large  stellate  motor  neurones  lying  to  the  inner 
side  of  the  emerging  facial  root-fibres. 

The  ventral  part  of  the  inner  area  and  the  adjoining  portion  of  the 
middle  one  are  occupied  by  the  field  of  the  median  Jillet,  which,  at  the  level 
under  consideration,  no  longer  lies  with  its  long  axis  directed  dorso-ventrally, 
but  approximately  horizontal  (Fig.  336).  The  tract  now  appears  as  a  com- 
pressed and  modified  oval,  with  the  thicker  inner  end  close  to  the  raphe  and 
the  tapering  outer  one  near  the  superior  oli\'e.  The  posterior  longitudinal 
fasciculus  is  seen  as  a  compact  strand,  immediately  beneath  the  gray  matter 
of  the  ventricular  floor  and  at  the  side  of  the  raphe.  Within  the  outer 
area,  lateral  to  the  facial  fibres,  appear  the  substantia  gelatinosa  and  the 
associated  descending  root  of  the  trigeminal  nerve.  Just  dorsal  to  the  latter, 
the  descending  vestibular  root  lies  close  to  the  inner  side  of  the  rest  if  or  m  body. 

THE   CEREBRAL    PEDUNCLES. 

That  part  of  the  brain-stem  which  encloses  the  Sylvian  aqueduct  corre- 
sponds approximately  with  the  morphological  division  of  the  brain  known 
as  the  mesencephalon.  The  latter  includes  two  main  subdivisions:  (i)  the 
smaller  dorsal  part,  the  quadrigeminal  plate,  which  roofs  the  Sylvian  aque- 
duct and  bears  the  corpora  quadrigemina,  and  (2)  the  much  larger  ventral 
part  made  up  by  the  cerebral  peduncles. 

The  latter,  also  called  the  cerebral  crura,  are  fused  dorsally  into  a  con- 
tinuous tract,  the  tegmentum ,  which  contributes  the  side-walls  and  floor  of 
the  Sylvian  aqueduct  and  blends  with  the  overlying  quadrigeminal  plate. 
Ventrally  the  peduncles  are  unfused  and  appear  on  the  inferior  surface  of 
the  brain  (Fig.  327)  as  two  robust  diverging  stalks,  enclosing  the  lower 
half  of  the  interpeduncular  space.  In  transverse  sections  (Fig.  339),  each 
stalk  is  seen  to  include  a  ventral  zone  composed  of  cross-cut  nerve-fibres, 
the  crusta,  and  a  crescentic  area  of  deeply  pigmented  gray  matter,  the  sub- 
stantia nigra,  that  separates  the  crusta  from  the  tegmentum. 

Disregarding  the  se\eral  small  nuclei,  the  nuclei  of  the  corpora  quadri- 
gemina and  the  red  nuclei,  the  gray  matter  within  the  mesencephalon  is 
disposed  as  three  tracts  that  extend  its  entire  length.  These  are  the  tubular 
mass  of  the  coitral  gray  matter,  which  surrounds  the  aqueduct,  and  the  two 
crescentic  columns  of  the  substantia  nigra,  which  subdi\'ide  the  peduncles 
into  the  tegmental  and  crustal  parts. 

The  central  gray  matter  {stratum  grisea  centrale)  encloses  the 
Sylvian  aqueduct  and  contains  numerous  irregularly  scattered  nerve- 
cells  of  uncertain  form  and  size  and,  along  its  ventral  border,  the  nuclei  of 
the  oculomotor  and  trochlear  ?ierves ;  within  its  lateral  parts  lie  the  nuclei 
from  which  proceed  the  fibres  of  the  mesencephalic  roots  of  the  trigeminal 
nerves. 


292  NORMAL    HISTOLOGY. 

The  substantia  nigra  owes  its  dark  color  to  the  deep  pigmentation 
of  its  numerous  nerve-cells.  The  latter  are  of  medium  size  and  of  various 
form,  spindle-shaped  elements,  interspersed  with  some  stellate  and  a  few 
pyramidal  ones,  predominating.  They  enclose  accumulations  of  dark  brown 
pigment  that  render  the  cells  unusually  conspicuous.  The  concave  dorsal 
margin  of  the  pigmented  crescent  is  continuous  and  even,  but  the  con\ex 
boundary  is  broken  into  irregular  scallops  by  processes  of  gray  matter  that 
penetrate  the  subjacent  crusta. 

The  crusta,  the  bold  sickle-shaped  field  that  occupies  the  most  ventral 
part  of  the  peduncle  (Fig.  340),  consists  chiefly  of  longitudinal  fibre-tracts, 
which  are  passing  from  the  cerebral  cortex,  by  way  of  the  internal  capsule, 

'3 

\ 


-     A-    7  r  "^>  ■ 

5  456  7 

Fig.  339. — Section  across  brain-stem  (mid-brain)  at  level  E,  Fig.  329.  i,  substantia  nigra,  separating 
crusta  from  tegmentum  ;  2.  crusia  of  cerebral  peduncle;  3,  superior  cerebellar  peduncle  and  (9)  its  decus- 
sation ;  4,  fjart  of  pons  ;  5,  interpeduncular  space  ;  6,  substantia  nigra  ;  7,  motor  tracts  in  crusta  ;  S,  stratum 
intermedium;  10,  mesial  fillet ;  11,  fountain  decussation  ;  12,  central  gray  substance;  13,  Sylvian  aque- 
duct; 14,  inferior  colliculus  of  corpora  quadiigemina  ;  15,  inferior  brachium;  16,  posterior  longitudinal 
fasciculus;  17,  tegmental  field.     X  3.     (Preparation  by  Professor  Spiller.) 

to  lower  levels  in  the  brain-stem  and  the  spinal  cord.  The  longitudinal  fibres 
are  separated  into  bundles  by  the  invasion  of  numerous  strands  from  the 
fibre-complex  known  as  the  stratum  i7itermedium,  which  lies  along  the 
ventral  border  of  the  substantia  nigra.  The  fibres  of  the  crusta  include 
three  general  sets:  (i)  the  cortico-pontile  fibres,  passing  from  the  cells  of 
the  cerebral  cortex  to  the  cells  of  the  pontile  nucleus  as  links  in  the  cortico- 
cerebellar  paths;  (2)  the  cortico-bulbar  fibres,  passing  as  axones  from  the 
motor  neurones  of  the  cerebral  cortex  to  the  nuclei  of  motor  fibres  origi- 
nating within  the  pons  and  medulla;  (3)  the  'corticospinal fibres,  passing  as 
axones  from  the  cerebral  motor  neurones,  through  the  ventral  tracts  of  the 
brain-stem,  into  the  pyramidal  tracts  of  the  spinal  cord,  to  end  around  the 
radicular  cells. 

The  tegmentum  of  the  mid-brain  includes,  as  seen  in  transverse  sec- 
tions (Fig.  339),  the  area  bounded  by  the  corpora  quadrigemina  behind 


THE   CEREBRAL    PEDUNCLES.  293 

and  the  crescents  of  the  substantia  nio;ra  in  h-ont.  In  the  vicinity  of  the 
central  gray  matter,  the  tegmentum  consists  chiefly  of  a  reticular  foimdation 
resembling  the  formatio  reticularis  seen  at  lower  levels.  This  substance  Ls 
produced  by  the  intermingling  of  transverse  or  arcuate  and  longitudinal 
fibres,  together  with  a  meagre  amount  of  gray  matter,  with  irregularly  dis- 
tributed cells,  that  fills  the  interstices  between  the  strands  of  fibres.  The 
more  lateral  and  ventral  parts  of  the  tegmentum  are  occupied  to  a  large 
extent  by  the  prominent  fibre-tracts  belonging  to  the  mesial  and  lateral 7?//<:'/.s- 
and  to  the  superior  ccrclnilar  peduncles,  or  by  collections  of  gray  matter,  the 

red  nuclei. 

The  nucleus  ruber,  or  nucleus  tegmcnti  (Fig.  340),  is  of  ovoid  form 
and  reddish  tint,  when  fresh,  and  consists  of  a  complex  of  gray  matter  and 
nerve-fibres.     The  latter  preponderate  below,  where  the  red  nucleus  receives 


Fig.  340.— Section  across  brain-stem  (mid-brain)  at  level  F,  Fig.  329.  i,  2,  root-fibres  of  oculomotor 
nerve;  3,  interpeduncular  space;  4,  red  nucleus;  5,  substantia  nigra;  6,  crusla  of  cerebral  peduncle;  7, 
stratum  intermedium  ;  8,  lateral  geniculate  body  ;  9,  fillet  fibres;  10,  superior  brachium  ;  11,  median  genic- 
ulate body  and  (12)  ils  nucleus;  13,  optic  fibres;  14,  pulvinar  of  optic  thalamus;  15,  stratum  zonale  of 
(16)  superior  colliculus;  17,  Sylvian  aqueduct;  18,  sup.  colliculus;  19,  central  gray  substance;  20,  22, 
nucleus  of  oculomotor  nerve;  21, posterior  longitudinal  fasciculus;  23,  red  nucleus.  X  aJi-  (Preparation 
by  Professor  Spiller.) 

the  fibres  of  the  superior  cerebellar  peduncle,  but  are  much  less  numerous 
above,  since  many  fibres  come  to  an  end  around  the  rubral  neurones. 
These  elements  are  very  variable  in  size  (20-60  //-)  and  shape,  but  are  mostly 
irregularly  triangular  or  stellate.  The  red  nuclei  not  only  constitute  im- 
portant stations  in  the  path  connecting  the  cerebellum  with  the  spinal  cord 
{cerebello-rubro-spinal  Jibrcs),  but  also  probably  contribute  links  in  the 
chain  uniting  the  cerebral  cortex  with  the  cord  (cerebro-rubro-spinal  Jibres) . 
While  some  of  the  constituents  of  the  superior  cerebellar  peduncle  pass 
around  the  red  nucleus  and  continue,  as  cerebello-thalamic  Jibres,  uninter- 
ruptedly to  the  optic  thalamus,  the  majority  of  the  fibres  of  the  peduncle 
end  around  the  neurones  of  the  nucleus,  from  which  then  proceed  axones 
as  the  rubro-tlialamic  Jibres.  It  is  important  to  remember  that,  in  a 
general  way,  the  ventral  part  of  the  brain-stem  transmits  the  great  efferent 
or  motor  paths,  while  within  the  dorsal  or  tegmental  part  ascend  the  chief 
afferent  or  sensory  tracts.  The  posterior  longitudinal  fasciculus  serves  as  an 
association-path  linking  together  the  nuclei  of  the  cranial  nerves. 


294 


NORMAL    HISTOLOGY. 


THE   CEREBELLUM. 

The  cerebellum  occupies  the  posterior  fossa  of  the  skull  and  lies  behind 
the  pons  and  medulla  and  the  fourth  ventricle  (Fig.  328).  By  means  of  its 
three  peduncles — inferior,  middle  and  superior — the  cerebellum  is  connected 
with  the  medulla,  the  pons  and  the  mid-brain  respectively.  Its  surface  is 
divided  by  the  deeper  fissures  into  lobides,  each  of  which  is  subdivided  by 
shallower  clefts  into  narrow  tracts,  \}aQ.  folia,  from  2-4  mm.  wide,  that,  within 
a  given  lobule,  in  a  general  way  parallel  one  another. 

With  the  exception  of  where  the  robust  peduncles  enter  the  hemispheres 
and  immediately  above  the  dorsal  recess  of  the  fourth  ventricle,  the  cere- 
bellum is  everywhere  covered  by  a  continuous  sheet  of  cortical  gray  matter 
which  follows  and  encloses  the  subdivisions  of  the  medidlary  layer.  The 
latter,    as  exposed  in  sagittal  sections   (F'ig.    341),   appears  as  a  compact 


Sylvian  aqueduct         Corpora  quadrigemiiia 


Cerebral  peduncle— —__Sj,^ 


IV  ventricle 

Ponf 

Roof  of  ventricle 

Tertiary  limb 

Choroid  plexus 

Medulla 


Cerebellar  cortex 

of  of  ventricle 


mb 

Core  of  white  matter 

Secondary  limb 

Surface 
Folium 

Interfoliar  fissure 
Cortex 

Lobule 


Fig.  341. — Mesial  sagittal  section  of  brain-stem  and  cerebellum,  showing  fourth  ventricle  and  relations 
of  the  central  white  matter  and  its  limbs  to  the  continuous  sheet  of  gray  cortex. 

central  mass  of  white  matter,  from  which  stout  stems  radiate  into  the  various 
lobules.  From  these  primary  stems,  secondary  branches  penetrate  the  sub- 
divisions of  the  lobules,  and  from  the  sides  of  these,  in  turn,  smaller  tracts 
of  white  matter,  the  tertiary  branches,  enter  the  individual  folia.  Over 
these  ramifications  of  the  white  core,  the  cortical  gray  matter  stretches  as  a 
fairly  uniform  layer,  about  1.5  mm.  thick,  that  closely  follows  the  complexity 
of  the  folia  and  fissures.  The  resulting  arborization  and  the  contrast  between 
the  white  and  gray  matter,  best  seen  in  sections  passing  at  right  angles  to  the 
folia,  produce  a  picture  known  as  the  arbor  vitce  cerebelli. 

The  Cerebellar  Cortex. — The  cortical  gray  matter  (Fig.  342)  includes 
two  very  evident  strata — the  outer  and  lighter  molecular  layer  and  the  inner 
and  darker  granule  layer. 

The  molecular  layer  is  of  uniform  thickness,  about  .4  mm.,  and 
contains  three  varieties  of  nerve-cells:  the  Purkinje  cells,  the  basket-cells  and 
the  small  cortical  cells.  The  Purkinje  cells,  the  most  distinctive  elements 
of  the  cerebellum,  occupy  the  deepest  part  of  the  molecular  layer,  where 
they  are  disposed  in  a  single  row  along  the  junction  of  the  outer  and  inner 
layers.      The  cells  are  more  numerous  and  closely  placed  upon  the  summit 


THE   CEREBELLUM.  295 

of  the  folium  than  along  the  fissures,  in  which  latter  situation  they  are  often 
less  typical.  They  possess  a  large  flask-shaped  body,  about  60  //.  in  diameter, 
from  the  pointed  and  outwardly  directed  end  of  which  usually  a  single  robust 
dendrite  arises.  The  chief  process,  relatively  thick  and  very  short,  divides 
into  two  branches,  which  at  first  diverge  and  run  more  or  less  horizontally 
and  then  turn  sharply  outwards  to  assume  a  course  vertical  to  the  surface  and 
to  undergo  repeated  subdivision.  The  arrangement  of  the  larger  dendrites 
is  very  striking  and  recalls  the  branching  of  the  antlers  of  a  deer.  The 
smaller  processes  arise  at  varying  and  often  acute  angles,  the  completed 
branching  resulting,  as  seen  in  silver  preparations  (Fig.  343),  in  an  arbor- 
ization of  astonishing  richness  and  extent,  that  may  reach  almost  to  the  outer 
boundary  of  the  molecular  layer.      The  dendritic  ramification  of  each  cell  is 

—Molecular  layer 

__-  Granule  layer 

-  White  matter 

y       ■■•.■                                            —        ^ 
f^'''  Cells  of  Purkinje 


Central  limb  of  while  matter 
Fig.  342. —Section  across  cerebellar  folium,  showing  relations  of  cortex  to  underlying  white  matter.     X  lo. 

limited,  however,  to  a  narrow  zone  extending  across  the  folium  and,  hence, 
when  examined  in  sections  cut  parallel  with  the  plane  of  the  folium,  the 
expansions  of  these  cells  are  found  to  be  confined  to  tracts  separated  by 
zones  of  the  molecular  layer  that  are  uninvaded  by  the  dendrites.  The 
axone  of  the  Purkinje  cell  arises  from  the  rounded  basal  or  deeper  end  of 
the  body  and  at  once  enters  the  subjacent  granule  layer,  which  it  traverses 
to  gain  the  white  medullary  substance  of  the  folium.  During  their  course 
the  axones  give  off  a  few  recurrent  collaterals  that  lose  themselves  within  the 
granule  layer  or  end,  in  part,  within  the  molecular  stratum. 

The  basket-cells  lie  chiefly  within  the  deeper  half  of  the  molecular 
layer.  They  possess  an  irregular  stellate  body  (10-20  /x),  from  which 
several  dendrites  radiate.  Their  chief  feature  is  the  axone,  which  extends 
across  the  folium  along  and  to  the  outer  side  of  the  Purkinje  cells.  During 
this  course,  the  axone  gives  off  from  three  to  six  collaterals  that  descend  to 
the  cells  of  Purkinje,  whose  bodies  they  surround  and  enclose  with  a  basket- 


296 


NORMAL   HISTOLOGY. 


like  arborization,  the  terminal  ramification  of  the  main  process  itself  ending 
in  like  manner.      By  this  arrangement  each  basket-cell  is  brought  into  close 

relation  with  several  of  the 
Purkinje  cells,  an  associa- 
tion probably  of  conse- 
quence in  insuring  coor- 
dination. 

The  small  cortical 
cells  include  two  varieties. 
One  is  represented  by  neu- 
rones of  about  the  size  of 
the  basket-cells,  or  slightly 
smaller,  provided  with  few 
dendrites  and  an  axone  dis- 
tinguished by  its  delicacy, 
great  length  and  tendency 
to  ramify  in  curves  or  even 
loops.  The  other  kind  is 
somewhat  smaller  and  pos- 
sesses axones  that  are  short 
and  soon  branch. 

The  granule  layer, 
of  a  rust-brown  tint  when 
fresh  and  deeply  colored 
in  stained  preparations,  is 
thickest  on  the  summit  of 
the  folia  and  thinnest  op- 
posite the  bottom  of  the 
While  sharply  defined  from  the  overlying  molecular  layer,   it  is 


Fig.   343. — Purkinje  cell  from  human  cerebellar  cortex ;  silver 
preparation  ;  a,  axone.     X  180. 


fissures. 


1 


-v<V 


'fl 


r 

( 

'i 

■     {".J 

,     ' 

Molecular  layer 


Granule  layer 


White  matter 


-Moss-fibres 

•Axones  of  Purkinje  cells 

-  Climbing  fibres. 


Fig.  344. — Diagrammatic  reconstruction  of  part  of  folium,  illustrating  relations  of  nerve-cells  and  fibres 
of  cerebellar  cortex ;  transverse  (left)  and  longitudinal  (right)  cut  surfaces  of  folium  are  shown  ;  a,  Pur- 
kinje cells  ;  b,  granule  cells ;  c,  small  cortical  cells  ;  d,  basket-cells  ;  <?,  large  association  cell  of  type  II. 

less  clearly  demarcated  from  the  medullary  substance,  with  which  it  some- 


THE  CEREBELLUM. 


297 


what  blends.     The  granule  layer  contains  two  varieties  of  nerve-cells — the 
granule  cells  and  the  large  stellate  cells. 

The  granule  cells  are  very  small  (7-10  //.)  and  numerous  and  so  closely 
packed  that  they  confer  a  distinct  density  on  the  stratum,  of  which  they  are 
the  chief  components.  They  are  provided  with  from  three  to  six  short 
radiating  dendrites  that  end  in  peculiar  claw-like  arborizations  (Fig.  344)  in 
relation  with  other  granule  cells.  Their  axones,  directed  towards  the  surface, 
enter  the  molecular  layer,  within  which  they  undergo  T-division  at  various 
levels,  corresponding  to  the  depth  at  which  the  cells  lie.  The  two  resulting 
branches  run  horizontally  and  lengthwise  in  the  folium,  that  is,  parallel  to 
the  surface  and  at  right  angles  to  the  plane  of  expansion  of  the  Purkinje 
dendrites,  through  whose  arborizations  they  find  their  way  and  with  which 
they  probably  come  into  close  relation.  The  axones  apparently  end  free 
and  without  arborizations. 

The  large  stellate  cells  are  present  in  varying  number,  but  are  never 
many.  They  lie  close  to  the  outer  limit  of  the  granule  layer  and  possess  a 
cell-body  of  uncertain  and  irregular  form,  from  30-40  [x  in  diameter,  from 
which  usually  several  richly  branched  dendrites  pass  in  different  directions, 
but  largely  into  the  molecular  layer.  The  a.xone  is  most  distinctive,  since 
very  soon  after  leaving  the  cell-body  it  splits  up  into  an  arborization  of 
unusual  extent  and  complexity,  which,  however,  is  confined  to  the  granule 
layer.  These  cells,  therefore,  belong  to  type  II  (page  66).  Since  their 
processes  are  brought  into  intimate  relation  with  a  number  of  other  neurones, 
these  elements  are  probably 
association  cells.  Additional 
nervous  elements  within  the 
granule  layer,  few  in  number 
and  fusiform  in  outline,  are 
described  as  the  solitary  cells, 
concerning  which  little  is 
known. 

The  cortical  nerve- 
fibres  include  three  chief 
varieties.  ( i )  The  axones  of 
Pio'kinje  cells  contribute  an 
inconsiderable  portion  of  the 
fibres  passing  between  the  cer- 
ebellar cortex  and  medullary 
substance.  They  end,  for  the 
most  part,  in  the  dentate 
nucleus  within  the  white  core 
near  the  root  of  the  superior  cerebellar  peduncles,  some  probably  terminating 
in  the  smaller  internal  nuclei  and,  perhaps,  in  relation  with  the  cells  of  the 
vestibular  and  inferior  olivary  nuclei.  (2)  The  so-called  moss-fibres ^  which 
ascend  from  the  medullary  into  the  granule  layer,  within  the  latter  repeatedly 
branch  and  bear  moss-like  tufts  that  end  in  large  part  in  irregular  small  masses 
of  stainable  substance,  known  as  the  eosin-bodies,  that  lie  between  the  granule 
cells  (Fig.  345).  According  to  Cajal,  these  bodies  are  formed  by  the  intri- 
cate interlacement  of  the  terminal  ramifications  of  the  afferent  axones  and  the 
dendrites  of  the  granule  and,  perhaps,  also  of  the  axones  of  the  Golgi  cells. 
In  view  of  their  convoluted  complexity  they  have  been  called  the  glomeruli 
of  the  cerebellum.  Other  filaments  of  the  moss-fibres  are  continued  into 
the  molecular  layer,  bending  horizontally  and  repeatedly  branching.      (3) 


Eosin-body 


Eosin-bodv 


Fig.   34S. — Portion   of   granule   layer  of   young  cerebellum, 
showing  eosin-bodies  and  nerve-fibres.     X  220. 


298  NORMAL   HISTOLOGY. 

The  climbing-fibres  ascend  through  the  granule  to  the  molecular  layer, 
where  their  tendril-like  ramifications  entwine  and  cling  to  the  principal  den- 
drites of  the  Purkinje  cells.  Additional  fibres  within  the  granule  layer  are, 
e\idently,  the  axones  of  the  granule  cells  and  the  collaterals  of  the  cells  of 
Purkinje,  whilst  a  large  proportion  of  the  fibres  within  the  molecular  layer 
are  the  ramifications  of  the  axones  of  the  granule,  the  basket  and  the  smaller 
cortical  cells. 

The  neuroglia  forms  a  supporting  framework  of  considerable  density 
within  both  the  cortex  and  the  medulla.  In  preparations  colored  with 
nuclear  stains,  the  neuroglia  cells  are  conspicuous  within  the  granule  layer, 
to  whose  numerous  small  nuclei  they  materially  contribute.  A  number  of 
relatively  large  neuroglia  cells  lie  within  the  outer  part  of  the  molecular 
layer,  near  the  Purkinje  cells;  in  addition  to  short  and  irregular  centrally 
directed  processes,  these  elements  give  ofi  brush-like  groups  of  fibres  which 
penetrate  the  molecular  layer,  seldom  branching,  as  far  as  the  surface  of  the 
folium,  where  they  end  in  minute  triangular  expansions.  By  the  apposition 
of  the  latter  a  delicate  subpial  condensation,  a  sort  of  limiting  membrane,  is 
produced.  The  radial  disposition  of  the  neuroglia  fibres,  as  well  as  of  the 
Purkinje  dendrites,  climbing  fibres  and  larger  blood-vessels,  confer  upon 
the  molecular  layer  a  vertical  striation.  Other  neuroglia  cells,  stellate  with 
spider-like  radiating  fibres,  occupy  all  levels  of  the  molecular  layer;  similar 
cells,  with  more  extended  processes,  occur  between  the  nerve-fibres  of  the 
medullary  substance. 

The  Internal  Nuclei. — In  addition  to  and  unconnected  with  the 
cerel^ellar  cortex,  four  paired  masses  of  gray  matter,  the  internal  nuclei — 
one  of  considerable  size  and  the  others  small — lie  embedded  within  the 
white  matter. 

The  nucleus  dentatus,  or  corpus  dentatiim,  the  largest  and  most 
important  of  the  internal  nuclei  (Fig.  346),  consists  of  a  plicated  sac  of  gray 
matter,  enclosing  nerve-fibres,  and  resembles  in  many  respects  the  inferior 
olivary  nucleus.  It  lies  within  the  anterior  part  of  the  median  half  of  the 
hemisphere  and  measures  from  15-20  mm.  in  its  longest  dimension. 

Of  the  other  paired  internal  collections  of  gray  matter — the  nucleus 
fastigii,  the  nucleus  emboliformis  and  the  nucleus  globosus — the  nucleus 
fastigii,  or  the  roof-nucleiis,  is  the  best  defined.  It  is  an  egg-shaped  mass, 
about  10  mm.  long,  and  lies  within  the  core  of  the  worm  (the  tract  connecting 
the  hemispheres)  close  to  the  mid-line  and  to  its  fellow  of  the  opposite  side. 

The  nucleus  emboliformis  and  the  nucleus  globosus  are  irregular 
masses  of  gray  matter  lying  between  the  dentate  and  roof-nuclei,  with  which 
they  are  respectively  united,  as  well  as  with  each  other. 

In  structure  the  internal  nuclei  differ  markedly  from  the  cerebellar 
cortex,  since  in  the  main  they  are  composed  of  irregularly  disposed  nerve- 
cells  of  one  kind,  interspersed  with  nerve-fibres.  The  dentate  nucleus  con- 
tains cells  (20-30  //),  angular  or  stellate  in  outline  and  pigmented.  Their 
processes  are  usually  so  disposed  that  the  axones  pass  into  the  medullary 
substance  enclosed  by  the  plicated  lamina  and  the  dendrites  into  the  sur- 
rounding white  matter  of  the  hemisphere.  Numerous  fibres  enter  the 
dentate  body  from  without,  many  being  the  axones  of  Purkinje  cells,  and 
ramify  within  the  folded  sheet  of  gray  substance.  Since  the  nuclei  emboli- 
formis and  globosus  are  only  incompletely  separated  parts  of  the  dentate 
nucleus,  their  structure  corresponds  closely  with  that  of  the  chief  mass. 
The  roof-miclei,  on  the  contrary,  possess  cells  of  niuch  larger  size  (40-80//.), 
more  rounded  form  and  greater  uniformity  in  tint,  although  they  are  distinctly 


THE   CEREBRUM.  299 

pigmented.  Numerous  strands  of  nerve-fibres  suljdi\ide  the  nucleus  into 
secondary  areas,  while  some  large  transversely  coursing  bundles  establish 
a  decussation  with  the  roof-nucleus  of  the  opposite  side.      Notwithstanding 


A 


\\v:. 


14_L_ 


_  7 


) 


"*        H.  t    A         A        ~^ 


Fig.  346. — Transverse  section  through  fourth  ventricle  and  surrounding  parts  of  brain-stem  and  of 
cerebellum  and  its  internal  nuclei  ;  i,  pyramidal  tracts;  2,  mesial  fillet;  3,  posterior  longitudinal  fascic- 
ulus; 4,  choroid  plexus;  5,  cerebellar  folia;  6,  medulla  ;  7,  fourth  ventricle;  S,  nucleus  dentatus  ;  9, 
inferior  worm;  10,  nucleus  fastigii  or  roof-nucleus;  11,  nucleus  globosus  ;  12,  nucleus  emboliformis  ;  13, 
nucleus  dentatus;  14,  choroid  plexus;  15,  restiform  body;  16.  fibres  of  vagus  nerve;  17,  spinal  root  of 
fifth  nerve;  18,  lateral  recess  ;  19,  olivary  nucleus.     X  3^-     (Preparation  by  Professor  Spiller.) 

their  small  relative  size,  the  dentate  and  roof-nuclei  are  important  stations, 
since  from  their  cells  arise  the  fibres  composing  the  greater  part  of  the 
superior  cerebellar  peduncle.  The  dentate  nuclei  are  of  additional  interest 
as  the  destination  of  many  axones  from  the  Purkinje  cells. 

THE   CEREBRUM. 

The  cerebrum — the  "great  brain"  as  distinguished  from  the  cerebellum 
— comprises,  in  a  general  way,  the  large  hemispheres  and  the  parts  surround- 
ing the  third  ventricle.  With  the  exception  of  where  the  peduncles  enter 
and  the  portion  of  the  mesial  surface  below  the  great  bridge,  the  corpus  callo- 
sum,  connecting  the  hemispheres,  the  cerebrum  is  everywhere  invested  with  a 
continuous  sheet  of  gray  matter,  the  cerebral  cortex.  On  account  of  the 
complex  convolutions  of  the  surface,  the  cortex  is  thrown  into  conspicuous 
folds  that  mark  the  convolutions  and  the  intervening  fissures  and  enclose  the 
penetrating  tracts  of  white  matter  prolonged  from  the  general  medullary  sub- 
stance. The  cortical  sheet  \aries  in  thickness  not  only  in  the  same  area, 
being  thicker  over  the  summit  than  along  the  sides  of  the  convolutions  or  at 
the  bottom  of  the  fissures,  but  in  different  regions.  Its  average  thickness  is 
about  3  mm. ,  but  where  it  borders  the  upper  end  of  the  Rolandic  fissure, 
this  increases  to  almost  5  mm. ,  whilst  over  the  occipital  poles  the  thickness 
of  the  cortex  is  reduced  to  about  2  mm.  The  entire  superficial  extent  of  the 
cortex  has  been  estimated  to  be  about  2000  sq.  cm. ,  of  which  scarcely  one 
third  is  exposed  surface,  the  remainder  being  sunken.  According  to  Don- 
aldson, the  cortex  contributes  about  one  half  of  the  weight  of  the  brain. 


300 


NORMAL    HISTOLOGY. 


Subpial  layer 
Tangential  fibres 

Stratum  zonale 


Layer  of  small 
pyramidal  cells 


The  essential  histological  elements  of  the  cerebral  cortex  are  the  nerve- 
cells  and  the  nerve-fibres.  The  importance  of  the  former  is  evident  when 
their  three-fold  activity  is  recalled — ( i )  as  receptors  of  afferent  or  corticipetal 
impulses,  (2)  as  distributors  of  the  impressions  so  received  to  other  parts  of 
the  brain,  and  (3)  as  originators  of  efferent  or  corticifugal  impulses  which 
control  the  nuclei  from  which  immediately  arise  the  motor  nerves.      No  sin- 

^  gle  method  of  preparation  suffices 

to  display  satisfactorily  both  groups 
of  structural  elements,  for  when 
stains  are  employed  which  best 
bring  out  the  cells,  the  fibres  are  in- 
adequately shown;  and,  conversely, 
when  methods  adapted  for  the 
demonstration  of  the  fibres  are  fol- 
lowed, the  cells  are  but  imperfectly 
displayed.  It  is  advantageous, 
therefore,  to  study  the  histological 
details  of  the  brain  by  more  than  a 
single  method,  combining  the  re- 
sults obtained  by  the  use  of  cellular 
stains  with  those  yielded  by  proced- 
ures exhibiting  the  fibres.  Among 
the  latter,  the  well-known  method 
of  Weigert,  or  its  modifications, 
has  been  of  great  service  in  extend- 
ing our  knowledge  concerning  the 
various  fibre-tracts.  The  methods 
of  silver-impregnation  introduced 
by  Golgi,  although  not  producing 
true  staining  but  only  incrustations 
on  the  cell  and  its  processes,  have 
materially  advanced  our  knowledge 
concerning  the  form  of  the  cell- 
bodies  and  the  number  and  extent 
of  the  processes  of  the  neurones. 

While  varying  as  to  details  in 
different  regions,  the  cerebral  cor- 
tex presents  a  general  plan  of  struct- 
ure which  may  be  considered:  (a) 
in  relation  to  the  nerve-cells  and  (d) 
in  relation  to  the  nerve-fibres. 
The  Nerve-Cells  of  the  Cortex. — When  sections  cut  perpendicular 
to  the  surface  of  the  convolution  are  stained  with  basic  stains  (Fig.  348)  or 
prepared  after  silver  impregnation  (Fig. '349),  the  cerebral  cortex  exhibits 
four  principal  layers,  which,  from  without  inwards,  are:  (i)  the  stratum 
zonale,  (2)  the  layer  of  small  pyramidal  cells,  (3)  the  layer  of  large  pyram- 
idal cells,  and  (4)  the  layer  of  polymorphic  cells.  Although  each  presents 
characteristics  which  are  distinctive,  with  the  exception  of  the  junction  be- 
tween the  first  and  second  layers,  where  the  change  is  well  defined,  no  sharp 
demarcation  separates  the  strata,  each  passing  insensibly  into  the  adjoining- 
layer.  Neither  are  the  modifications  which  distinguish  the  cortex  of  certain 
regions  abruptly  assumed,  one  type  of  cortical  structure  being  gradually 
replaced  by  another  without  sudden  transition. 


Outer  stripe  of 
Eail  larger 

Layer  of  large 
pyramidal  cells 


Layer  of  poly- 
morphic cells 


Medullary  fibres 


Fig.  347. — Diagram  illustrating  composition  of  cere- 
bral cortex;  cells  in  right  half,  fibres  in  left  half  of 
figure  ;  A,  B,  large  and  small  pyramidal  cells  ;  C,  poly- 
morphic cells  ;  £),  cell  of  Martinotti ;  £,  cell  of  type 
II;  /s  association  cell;  i,  i,  fibres  passing  to  cortex; 
2,  2,  2,  fibres  passing  from  cortex  ;  JV,  neuroglia  cells. 


THE   CEREBRUM. 


301 


i'^'J 


stratum 
zonale 


Small  pyra- 
m.idal  cells 


The  stratum  zonale,  also  known  as  the  molecular  stratum,  underlies 
the  pia  and  measures  about  .25  mm.  in  thickness.  The  layer  contains  few 
nerve-cells  and  appears  subdivided  into  {a)  a  narrow  peripheral  zone,  from 
10-30  [i.  in  width,  composed  of  a  subpial  condensation  of  neuroglia  and  (/^) 
a  deeper  zone  characterized  by  numerous  fibres  or  processes,  which  course 
parallel  to  the   surface,    and    a  meage 

number  of   nerve-cells  whose  most  dis-     :~--^'^'- —  -  •   -  ^^^^^sapia 

tinctive    representatives  are  small  fusi-  -^ 

form  elements  {CaJaP s  cells)  provided 
with  long  tangentially  directed  proces- 
ses. The  latter  give  off  short  collaterals, 
which  ascend  towards  the  surface,  and 
intermingle  with  the  numberless  terminal 
filaments  derived  from  the  peripherally 
coursing  processes  of  the  pyramidal  and 
other  cells  lying  at  deeper  levels  and 
from  the  corticipetal  fibres  which  con- 
tinue from  the  white  core  of  the  gyrus 
into  the  outermost  layer  of  the  cortex. 

The  layer  of  small  pyramidal 
cells  is  marked  off  from  the  stratum 
zonale,  which  it  about  equals  in  thick- 
ness, with  some  distinctness  since,  in 
contrast  to  the  last-mentioned  zone,  it 
contains  very  many  cells.  These,  as 
indicated  by  the  name  of  the  stratum, 
are  of  small  size  (y—io/j.')  and  pyramidal 
form,  at  least  in  the  deepest  part  of  the 
layer.  In  the  superficial  part  the  cells 
are  rounded  or  irregularly  triangular, 
but  they  assume  the  distinctive  pyram- 
idal outline  as  they  approach  the  sub- 
jacent layer,  whose  elements  they  re- 
semble in  possessing  apical  and  lateral 
processes. 

The  layer  of  large  pyramidal 
cells  contains  the  most  distincti\-e  neu- 
rones of  the  cerebral  cortex.  It  measures 
usually  about  1.25  mm.  in  thickness, 
but  in  some  localities  much  more,  and 
blends  with  the  adjoining  layers  without 
sharp  boundaries.  The  cells  increase  in 
size  but  diminish  in  numbers  as  they  are 
traced  from  the  second  layer  inwards, 
the  largest  (20-40  ;j.  in  width)  and  most 
characteristic  lying  in  the  deepest  part 
of  the  stratum.  The  typical  pyramidal  cell  possesses  a  conical  body,  trian- 
gular in  section,  the  apex  of  which  is  continued  into  a  long  tapering  dendrite, 
the  apical  process,  which  extends  towards  the  periphery  for  a  variable  but 
usually  considerable  distance,  depending  upon  the  position  of  the  cell.  Upon 
gaining  the  stratum  zonale,  towards  which  the  apical  dendrite  is  always  direct- 
ed, the  process  breaks  up  into  a  number  of  end-branches  that  run  parallel  with 
the  surface  and  contribute  to  the  fibre-complex  of  the  outer  layer.     During  its 


Large  pyra- 
midal cells 


-^) 


m. 


Polymorphic 
cells 


Fig.  348. — Section  of  cerebral  cortex.    X  90. 


302 


NORMAL    HISTOLOGY. 


Small  pyramidal 
cells 


journey  to  the  surface,  the  apical  dendrite  gives  off  an  uncertain  number  of 
branches  that  continue  horizontally  and,  with  the  collaterals  and  similarly 
directed  processes  from  other  cells,  take  part  in  producing  the  feltvvork  giv- 
ing rise  to  a  thin  light  band,  known  as  the  outer  stripe  of  Baillarger.  From 
the  deeper  or  basal  surface  of  the  cell  arises  the  delicate  centrally  directed 
axone,  which,  penetrating  the  intervening  fourth  layer,  acquires  a  medullary 
coat  and  enters  the  white  core  of  the  convolution  as  one  of  the  component 
ner^'e-fibres.  The  axone  gives  off  one  or  more  collaterals  which,  after  a 
shorter  or  longer  course,  establish  relations  with  other  and  often  remote  cells. 

In  addition  to  the  two  chief  proc- 
esses, the  peripherally  directed 
apical  dendrite  and  the  centrally 
coursing  axones,  a  variable 
number — from  four  to  twelve 
— of  secondary  lateral  dendrites 
spring  from  the  basal  angles  of 
the  cell.  These  processes  usually 
divide  dichotomously,  each  suc- 
ceeding pair  of  branches  in  turn 
splitting  into  twigs,  until  the 
dendrite  is  resolved  into  an  end- 
brush  of  fibrillse  which  aid  in 
producing  an  intricate  feltvvork 
of  finest  threads.  Each  pyram- 
idal cell  contains  a  conspicu- 
ous spherical  or  ellipsoidal  nu- 
cleus, within  which  a  distinct 
nucleolus  is  usually  distinguish- 
able. The  cytoplasm  exhibits 
a  striation  and,  in  addition  to 
the  masses  of  tigroid  substance, 
the  Nissl  bodies,  a  mass  of 
brownish  pigment  granules. 
The  larger  pyramidal  cells  are 
surrounded  by  an  evident  peri- 
cellular lymph-space. 

The  layer  of  polymor- 
phic cells  includes  a  large 
number  of  small  nerve-cells, 
from  8-IO  ,a  in  diameter,  whose 
forms  vary  greatly,  irregular, 
spherical,  triangular,  stellate 
and  fusiform  elements  being  present.  Small  pyramidal  cells  are  also  often 
seen  within  this  layer.  In  contrast  to  dendrites  of  the  typical  pyramidal  cells, 
those  of  the  polymorphic  elements,  although  peripherally  directed,  do  not 
reach  the  stratum  zonale  but  end  before  gaining  the  outermost  layer.  Their 
axones  pass  into  the  subjacent  fibre-layer.  The  radial  disposition  of  the 
groups  of  fibres  within  the  deepest  stratum  of  the  cortical  substance,  limits 
the  polymorphic  cells  chiefly  to  the  interfascicular  areas,  within  which  the 
cells  consequently  appear  arranged  in  a  somewhat  columnar  order. 

Within  the  deeper  layers  of  the  cortex,  therefore  among  the  polymor- 
phic and  the  pyramidal  elements,  two  additional  varieties  of  nerve-cells  are 
encountered.     These  are  the  cells  of  Martinotti  and  the  cells  of  Golgi. 


Large  pyramidal 
cells 


Polymorphic 

cells 


Fig.  349. — Nerve-ceiis  of  cerebral  cortex,  after  silver  im- 
pregnation.    X  70.     (Preparation  by  Prof.  T.  G.  Lee. J 


THE   CEREBRUM. 


303 


Tan.srentiar 
fibre-layer 


Supra  radial 
felt-work 


Outer  stripe 
jf  Baillarger 


The  cells  of  Martinotti  are  of  small  size  and  triangular  or  spindle- 
form  in  outline  and  particularly  distinguished  by  the  unusual  direction  of 
their  axones.  These  processes  pass  towards  the  surface  and  within  the 
stratum  zonale  divide  into  branches,  which  are  continued  horizontally  in  the' 
feltwork  of  tangential  fibres.  As  in  other  parts  of  the  central  nervous  sys- 
tem, so  too  in  the  cerebral  cortex  there  is  found  a  sprinkling  of  Golgi's  cells 
of  type  II.  Although  both  dendrites  and  axones  of  these  cells  undergo 
elaborate  arborization,  the  axone  is  confined  to  a  limited  territory  in  the 
\icinity  of  the  cell  and,  therefore,  never  reaches  the  stratum  zonale. 

Neuroglia  cells  are  present  in  all  parts  of  the  cerebral  cortex  and, 
wWiIq  in  a  general  way  they  send  fibrils  in  all  directions  between  the  nervous 
elements,  which  they  then  support,  the  arrangement  of  the  fibrilhe  is  fairly 
definite  in  certain  strata.  Thus  with- 
in the  subpial  condensation  of  the 
neuroglia,  the  glia  cells  send  most 
of  their  processes  as  inwardly  directed 
brushes.  The  cells  within  the  deep- 
er part  of  the  cortex  give  ofi  their 
processes  in  two  chief  groups,  one 
extending  towards  the  periphery  and 
the  other  towards  the  white  core. 

The  Nerve-Fibres  of  the 
Cortex. — When  viewed  in  suitably 
stained  sections  cut  parallel  with  their 
general  course,  the  cortical  nerve- 
fibres  do  not  appear  as  a  uniform 
layer,  but  as  radially  disposed  bun- 
dles which  gradually  become  less 
distinct  as  they  traverse  the  cortex 
and  finally  disappear  at  about  the 
level  of  the  outer  border  of  the 
layer  of  large  pyramidal  cells.  The 
radial  fibres  are  partly  afferent  and 
partly  efferent.  The  corticifugal 
coinpo7ients,  which  predominate,  are 
largely  the  centrally  directed  axones 
of  the  pyramidal  and  the  polymor- 
phic cells  which  are  continued  as  the 
axis-cylinders  of  the  fibres  composing 
the  subcortical  white  matter.  The 
peripherally  coursing  axones  of  the 
cells  of  Martinotti  also  contribute  to 
the  production  of  the  fibre-radii. 
The  corticipdal  coiistitiients  of  these 
tracts  include  the  nerve-fibres  which 
are  derived  from  cells  situated  more 
or  less  remote  from  the  convolution 
in  which  the  fibres  (their  axones)  end. 


Interradial 
felt  work 


Radiil  fibres 


Fig.  350. — Section  of  cerebral  cortex  stained  to 
show  nerve  fibres  ;  the  cells  are  not  seen  but  lie  be- 
tween tlie  strands  of  fibres.     X  21. 

Such,  for  example,  are  the  thalamo- 


cortical and  the  tegmento-cortical  fibres,  as  well  as  the  many  commissural 
fibres  that  arise  in  the  opposite  hemisphere  and  cross  by  way  of  the  corpus 
callosum.  Although  for  the  most  part  the  corticipetal  fibres  end  at  various 
levels  in  arborizations  around  the  pyramidal  cells,  some  are  continued  into 
the  stratum  zonale  where  they  assist  in  producing  the  tangential  zone. 


304  NORMAL  HISTOLOGY. 

The  spaces  between  these  radial  bundles  are  occupied  by  a  delicate 
interlacement,  the  interradial  feltwork,  which  is  composed  in  large  part 
of  the  lateral  and  collateral  processes  of  the  cells.  Within  the  third  layer, 
the  horizontally  coursing  collaterals  and  processes  of  the  large  pyramidal 
cells  form  a  complex  of  unusual  intricacy,  which  condensation  gives  rise  to 
the  outer  stripe  of  Baillarger.  Beyond  the  outer  ends  of  the  radial  fibre- 
bundles,  the  intercellular  ground-work  is  occupied  by  a  second  delicate  inter- 
lacement of  processes  and  collaterals,  the  supraradial  feltwork;  while 
immediately  beneath  the  narrow  subpial  neurogliar  zone  innumerable  delicate 
terminal  fibrillae  course  horizontally  and  parallel  with  the  surface  and  con- 
stitute the  tangential  fibre-layer.  The  components  of  this  layer  are  the 
terminal  branches  of  the  dendrites  of  the  pyramidal  and  polymorphic  cells 
and  the  axones  of  the  cells  of  Mattinotti,  as  well  as  the  main  and  secondary 
processes  of  the  fusiform  elements  of  the  stratum  zonale. 

Local  Variations  in  the  Cerebral  Cortex.^ — While  in  the  main 
certain  features  are  common  to  the  cortex  wherever  well  developed,  more  or 
less  evident  variations  occur  in  different  localities.  Such  variations  are,  for 
the  most  part,  slight  and  depend  upon  the  size  and  number  of  the  nerve-cells 
and  the  richness  and  direction  of  the  nerve- fibres — changes  which  produce 
alterations  in  the  relative  proportions  of  the  strata.  The  width  of  the  stratum 
zonale  is  almost  constant  and  subject  to  little  modification,  being  usually  well 
defined  from  the  layer  of  small  pyramidal  cells.  The  layer  of  the  large 
pyramidal  cells,  on  the  contrary,  exhibits  considerable  variation,  either  in 
increased  thickness,  as  in  the  precentral  gyrus,  or  in  diminished  breadth,  as 
in  the  occipital  lobe.  The  layer  of  polymorphic  cells  is  fairly  uniform,  but 
within  the  precentral  convolutions  is  reduced  almost  to  disappearance, 
although  the  pyramidal  cells  of  the  superimposed  (third)  layer  are  here  of 
unusual  size.  Such  variations  in  the  histological  features  of  the  cortex  are 
probably  correlated  with  differences  in  the  function  of  its  various  regions, 
although  the  exact  relations  between  such  differences  are  in  many  cases  still 
obscure.  Disregarding  the  cortical  regions  which  are  profoundly  modified 
by  their  rudimentary  character,  such  as  the  olfactory  lobe,  apart  from  minor 
variations  in  details,  the  cortex  of  the  greater  part  of  the  frontal,  parietal, 
occipital,  temporal  and  limbic  lobes  and  of  the  insula  closely  corresponds  in 
its  structure.  That  of  the  motor  (Rolandic)  region,  of  the  calcarine  (visual) 
area  of  the  occipital  lobe,  and  of  the  hippocampus,  dentate  gyrus  and 
adjacent  part  of  the  hippocampal  gyrus,  however,  presents  very  evident 
modification. 

The  Rolandic  cortex  of  the  precentral  gyrus,  particularly  towards  the  upper  margin 
of  the  hemisphere,  of  the  paracentral  lobule  and  of  the  adjoining  part  of  the  postcentral 
gyrus — the  great  cortical  motor  area  of  the  hemisphere— is  distinguished  by  the  great 
breadth  of  the  layer  of  large  pyramidal  cells,  the  unusual  size  of  the  last-named  elements 
and  the  feeble  development  of  the  layer  of  polymorphic  cells.  The  pyramidal  cells 
collectively  tend  to  larger  size  as  the  upper  end  of  the  precentral  convolution  is  ap- 
proached and,  in  addition,  cells  of  extraordinary  dimensions  appear.  These  elements, 
the  giant  pyramidal  or  Betz'  s  cells,  reach  their  maximum  size  within  the  paracentral 
lobule,  where  some  attain  a  breadth  of  65  11  or  almost  double  that  of  the  pyramidal 
elements  in  other  regions.  The  giant  cells  are  further  distinguished  by  their  robust 
and  rounded  form,  their  distribution  in  small  groups  of  from  three  to  five  in  the  deeper 
layers  of  the  cortex,  and  the  exceptional  thickness  of  their  axones. 

The  Internal  Nuclei. — Embedded  within  the  white  matter  of  the 
cerebrum,  for  the  most  part  completely  separated  from  the  cortex,  lie  certain 
paired  masses  of  gray  matter  collectively  known  as  the  basal   ganglia. 


THE   CEREBRUM. 


305 


These  include:  the  two  parts  of  the  corpus  striatum,  the  caudate  and  leyiticular 
nuclei,  the  claustrum,  the  amygdaloid  nucleus,  and  the  thalamus.  Of  these, 
two,  the  corpus  striatum  and  the  thalamus,  will  be  briefly  described. 

The  Corpus  Striatum. — This  large  mass  of  gray  matter,  which 
extends  from  the  outer  wall  of  the  lateral  ventricle  to  almost  the  cortex, 
consists  of  two  parts,  the  caudate  nucleus  or  inner  division  and  the  lenticular 
nucleus  or  outer  division.  Although  continuous  in  front  and  below,  the  two 
parts  are  separated  almost  completely  by  the  intervening  important  tract  of 
white  matter,  the  internal  capsule,  which  is  the  great  pathway  for  nerve- 
tibres  between  the  cerebral  corte.x  and  the  lower  portions  of  the  brain. 

The  caudate  nucleus  is  an  elongated  comet-shaped  mass  of  gray 
matter,   well  seen   from  the  lateral  ventricle,  and   invested  throughout  the 


'^/'i^l'' 


Fig.  351. — Portion  of  oblique  frontal  section  of  cerebrum,  showing  the  ventricles,  the  basal  g:anglia 
(caudate  and  lenticular  nuclei  and  optic  thalami)  and  the  internal  capsule,  i,  fornix,  below  corpus 
callosum  (25) ;  2,  choroid  plexus  ;  3,  lateral  ventricle  ;  4,  stratum  zonale  ;  5,  caudate  nucleus  ;  6,  internal 
capsule,  knee;  7,  mesial  and  tS)  lateral  nucleus  of  thalamus;  9,  internal  capsule;  10,  putamen  and  (11) 
globus  pallidus  of  lenticular  nucleus;  12,  anleiior  pillars  of  fornix;  13,  lamina  terminalis;  14,  anterior 
commissure;  15,  olfactory  fibres;  16,  thalamo-tegmental  tract;  17,  globus  pallidus;  18,  putamen;  19, 
mammillo-thalamic  tract;  20,  external  and  (21)  internal  medullary  lamina  ;  22,  stria  terminalis  ;  23, caudate 
nucleus  ;  24,  striate  vein  ;  25,  corpus  callosum  ;  26,  cingulum  ;  27,  gyrus  callosus.  X  J-  (Preparation  by 
Professor  Spiller.) 

greater  part  of  its  periphery  by  a  dense  layer  of  fibres,  the  stratum  zonale, 
covered  on  its  ventricular  surface  by  the  cuboidal  ependymal  epithelium.  The 
nerve-cells  of  the  nucleus  are,  for  the  most  part,  rather  small  pigmented  and 
stellate  or  fusiform  and  belong  to  type  II,  with  short  a.xones.  Some  cells, 
however,  are  very  large  and  provided  with  long  axones  which  may  pass  into 
the  internal  capsule. 

The  lenticular  nucleus  lies  to  the  outer  side  of  the  internal  capsule 
and  is  subdivided  by  two  narrow  tracts  of  white  matter,  the  viedullary 
lamince,  into  three  segments.  Of  these  the  outer  one,  ^\\q.  putamen,  is  the 
darkest  and  corresponds  in  structure  closely  with  the  nucleus  caudatus. 
The  middle  and  inner  zones  together  constitute  the  globus  pallidus  and  are 
lighter  in  tint,  owing  to  the  excessive  number  of  pervading  bundles  of  nerve- 
fibres.     All  three  zones  of  the  nucleus  ienticularis  consist  of  gray  matter 


3o6 


NORMAL   HISTOLOGY. 


intermingled  with  many  nerve-fibres.  The  cells  of  the  globus  pallidus,  save 
for  their  somewhat  smaller  size  (20-40  ij.)  arid  a  lighter  pigmentation,  re- 
semble the  smaller  neurones  of  the  caudate  nucleus,  but  are  less  generally 
cells  of  type  II.  Large  neurones  (35-70  ,«)  are  common  in  the  putamen, 
having  a  slender  cell-body  and  giving  off  at  the  poles  two  or  more  dendrites. 
The  axone  often  arises  from  the  base  of  a  dendritic  process. 

The  Thalamus. — This  large  ovoid  ganglionic  mass  lies  between  the 
third  ventricle  mesially,  of  which  cavity  it  constitutes  the  lateral  wall,  and 
the  internal  capsule  laterally.  Its  free  dorsal  surface,  and  to  a  less  degree 
the  mesial  as  well,  is  covered  with  a  thin  layer  of  nerve-fibres,  the  stratiun 
zonale,  whilst  ventro-laterally  the  thalamus  is  separated  from  the  internal 
capsule  by  a  denser  layer  of  fibres,  the  exter?ial  inedullary  lamina.  The 
gray  matter  is  subdivided  into  two  general  parts,  the  lateral  and  the  mesial 
nucleus,  by  a  vertical  sheet  of  white  matter,  the  internal  medullary  lamina. 
The  periphery  of  the  lateral  nucleus  is  broken  up  by  numerous  fibres  passing 


Medullary  branch 


Larger  pial  artery 


White  matter 


Pia  within  fissure  -"^^^^ 
Fig.  352. — Injected  cerebral  cortex,  showing  capillary  supply  of  gray  and  white  substance.    X  i8. 

from  the  thalamus  into  the  internal  capsule  into  a  reticular  zone,  sometimes 
called  the  latticed  layer.  The  nerve-cells  of  the  thalamic  nuclei  include  three 
chief  varieties:  (i)  the  stellate  cells  (35-50  /.t),  distinguished  by  their  richly 
branched  dendrites  that  radiate  in  all  directions;  (2)  the  brush-cells,  so  called 
on  account  of  the  brush-like  expansions  of  their  dendrites,  spindle  or  trian- 
gular and  from  20-40  //  in  diameter;  and  (3)  the  polygonal  cells,  few  but 
large  (50-60  /-/.),  with  a  number  of  long  slender  tortuous  dendrites.  The 
thalamic  cells  are  by  no  means  uniformly  distributed,  but,  on  the  contrary, 
are  aggregated  into  larger  and  smaller  groups,  the  subsidiary  nuclei;  of 
these  eleven  are  distinguished  by  Cajal,  by  whom  they  are  described  as 
lying  in  three  general  planes  and  constituting  a  lateral,  an  intermediate  and 
an  inner  series.  Of  these  the  ventro-lateral  micleits  is  of  especial  impor- 
tance, since  it  probably  receives  the  afferent  fibres  of  the  great  sensory  fillet- 
tract.  In  a  general  way,  the  neurones  of  the  thalamus  may  be  regarded  as 
receptors  of  a  large  part  of  the  afferent  impulses  carried  to  the  cerebrum 
from  the  cerebellum,  brain-stem  and  spinal  cord,  such  impulses  being  dis- 
tributed by  the  thalamo-cojiical fibres,  the  axones  of  the  thalamic  cells,  to  the 


THE   CEREBRUM. 


307 


cerebral  cortex.  Since,  moreover,  the  cells  of  the  latter  send  cortico- 
tlialaiiiic  fibres  centrally,  the  intricate  character  of  the  connections  of  the 
thalamus,  which  also  gives  off  many  additional  fibres  to  lower  lying  levels, 
is  evident. 

Blood-Vessels  of  the  Brain. — The  arteries  supplying  the  brain 
are  derived  from  the  internal  carotid  and  vertebral  stems.  Their  immediate 
distribution  to  the  cerebral  and  cerebellar  cortex  is  everywhere  through  the 
agency  of  the  pia  mater,  within  which  the  larger  trunks,  after  frequent  anas- 
tomoses, give  off  the  abundant  small  end-arteries  that  penetrate  the  subja- 
cent nervous  substance.  On  entering  the  gray  matter,  these  cortical 
branches,  whose  general  course  is  parallel  to  one  another  and  at  right  angles 
to  the  surface,  break  up  into  rich  networks  of  capillaries  coming  into  direct 
relation  with  the  ner\'ous  elements.  The  vascular  supply  of  the  gray  matter 
is  more  generous  than  that  of  the  white  substance,  which  latter  receives, 
however,  in  addition  to  continuations  from  the  cortical  capillary  network,  a 
number  of  viedulla7y  branches.  These  contribute  few  twigs  to  the  gray 
substance  but  traverse  the  latter  and  have  their  chief  distribution  as  long- 
meshed  capillary  networks  within  the  white  matter.  The  arteries  are 
accompanied  by  connective  tissue  envelopes,  prolonged  from  the  pia  mater, 
which  enclose  ^\'\s\\q.?^\\\\\^  perivascnlar  lymph -spaces. 

The  cortical  veins  begin  in  the  white  matter  and  pass  through  the 
gray  sheet  to  reach  the  pia  mater,  within  whose  external  part  they  ramify, 
the  arteries  usually  lying  deeper.      The  larger  emergent  stems,  howex-er,  do 


Gray  matter 


White  matter 


F"i-  353- — Itijected  dentate  nucleus  of  cerebellum,  showing  rich  capillary  supply  of  plicated  gray  matter. 

X  20. 

not  follow  the  main  branches  of  the  cerebral  arteries,  but  converge  towards 
the  lines  of  the  principal  adjacent  dural  sinuses  into  which  they  open.  The 
cerebral  veins  are  among  those  possessing  little  or  no  muscular  tissue  and 
no  valves. 

True  lymphatics  are  found  neither  within  the  brain  nor  spinal  cord. 
Lymph-paths,  howe\'er,  are  represented  by  the  perivascular  sheaths  sur- 
rounding the  blood-vessels  within  the  nervous  substance;  these  tracts  com- 
municate with  the  subarachnoid  space.  The  pericellular  spaces  enclosing 
the  larger  nerve-cells,  as  well  as  uncertain  subpial  spaces  between  the  pia 
mater  and  the  surface  of  the  nervous  substance,  are  closely  related  to  the 
lymphatic  system,  although  not  directly  communicating  with  it. 


3o8 


NORMAL   HISTOLOGY. 


THE   PINEAL   BODY. 

The  pineal  body,  also  called  the  epiphysis  and  the  conarium,  is  a  cone- 
shaped  organ,  from  8-10  mm.  in  length,  attached  to  the  posterior  extremity 
of  the  roof  of  the  third  ventricle. 


Epithelial 

tissue"^"*''^*''-"- 


Calcareous  Jx- 
concretion 


tissue  septa     '^{^^    ^^,^1^  <^^fli,  jT^i'l^^'"  ^'^  «*^ 


Lenticular  area 
Retinal  area 


Pig.  354. — Section  of  pineal  body,  showing  general  structure  and  calcareous  concretions.    X  130. 

As  seen  in  sections  of  the  adult  human  organ,  its  structure  includes  a 
reticular  framework  of  vascular  connective  tissue  trabeculae,  the  meshes  of 

which  are  filled  with  round- 
ed or  elongated  epithelial 
cells,  which  often  contain 
brownish  pigment.  With 
the  exception  of  a  few 
nerve-fibres  in  the  anterior 
part,  probably  sympathetic 
in  origin  and  destined  for 
the  walls  of  the  blood-ves- 
sels, and  a  dense  network 
of  neuroglia-fibres  in  the 
under  part,  the  pineal 
body  contains  no  elements 
of  a  nervous  character, 
nerve-cells  being  absent. 
Very  commonly  the  adult 
organ  encloses  a  variable 
number  of  concretions, 
called  brain-sand  or  acer- 


Blood- vessel 


Diverticulum 

dividing  into 

tubules 


■L^- 


PyrdL 


P'G-  355- — Sagittal  section  of  pineal  organ  of  lizard  embryo.  X  i75- 


Villus  cerebri,  which  consist  of  laminated  masses  composed  of  calcium  car- 
bonate and  phosphate,  mingled  with  organic  material.  They  may  be  of 
microscopic  dimensions,  or  reach  the  size  of  a  millet  seed,  and  by  aggre- 
gation assume  a  mammillated  form. 


THE   PITUITARY    BODY. 


309 


The  signiiicance  of  the  pineal  body  in  man,  long  an  unsolved  riddle,  has 
been  shown  by  embryological  and  comparative  studies  of  the  organ  in  the 
lower  vertebrates,  especially  in  lizards,  to  be  that  of  a  very  imperfectly 
developed  and  greatly  modified  rudimentary  sense-organ.  In  certain 
lizards,  in  which  it  reaches  a  high  development,  the  pineal  body  is  a  flat- 
tened cup-shaped  organ  (Fig.  355)  connected  with  the  brain  by  a  stalk 
containing  ner\-e-fibres.  The  structural  resemblance  to  an  invertebrate 
visual  organ,  a  sort  of  lens  o\erlying  a  retina-like  layer,  suggested  a  possible 
similarity  of  purpose  in  the  higher  types.  The  organ  was  designated,  there- 
fore, the  pineal  cyi\  although  probably  in  no  existing  animal  a  functioning 
structure.  The  embryonic  relations  in  many  reptiles  are  most  suggestive  of 
the  significance  of  tlie  jiineal  body  as  a  rudimentary  sense-organ,  although  not 
necessarily  an  eve. 

THE   PITUITARY   BODY. 

The  pituitary  body,  or  hypophysis  cerebri^  is  attached  to  the  dependent 
tip  of  the  infundlbulum,  the  narrow  funnel-like  projection  from  the  floor  of 
the  third  ventricle.  It  is  of  flattened  oval  form,  somewhat  mushroom- 
shaped,  and  measures  about  12  mm.  in  the  transverse  and  about  7  mm.  in 
the  sagittal  diameter.  The  pituitary  body  includes  two  entirely  distinct 
parts,  the  antetior  and  posterior  lobes,  which  differ  both  in  origin  and  struct- 


Interlobar  septum. 


^f^T^ 


Posterior  or  cerebral  lobe 


^3 — Blood-vessel 

\ 


• Anterior  or 

;        oral  lobe 


Connectivetissue^ 
trabecula 


.Capsule 


Fig.  356. — Horizontal  section  of  pituitary  body,  showing  relation  of  anterior  (oral)  and  posterior 

(cerebral)  lobes.     X  7. 

ure.  The  former  is  derived  as  an  outgrowth  from  the  roof  of  the  primitive 
oral  cavity,  while  the  latter  is  de\'eloped  as  a  tubular  evagination  from  the 
floor  of  the  second  brain-vesicle,  the  diencephalon. 

The  Anterior  Lobe. — The  anterior  and  glandular  division,  which 
constitutes  the  major  part  of  the  hypophysis,  is  surrounded  by  a  robust 
fibrous  capsule  that  is  continuous  with  the  thinner  investment  enclosing  the 
posterior  lobe.  From  the  deeper  surface  of  the  capsule,  as  well  as  from  a 
condensation  of  connective  tissue  on  each  side  of  the  mid-line  that  marks  the 
position  of  large  blood-vessels,  fine  processes  extend  inwards  and  form  a  deli- 
cate supporting  fibrous  reticulum,  rich  in  capillaries,  whose  meshes  are  filled 
with  spherical  or  cord-like  masses  of  cuboidal  or  polygonal  epithelial  cells. 
The  latter  are  apparently  of  two  kinds — the  smaller  chief  cells  (30-40  fi), 


3IO 


NORMAL    HISTOLOGY. 


which  color  best  with  basic  and  only  slightly  with  acid  stains,  and  the  larger 
chromophile  cells  (50-So  11),  so  named  because  of  their  affinity  for  certain 
acid  dyes  (eosin).  The  two  varieties  of  cells  are  intermingled  in  the 
anterior  lobe  without  definite  arrangement  and,  perhaps,  differ  merely  in 
functional  condition,  the  two  varieties  being  essentially  identical. 

The  aggregations  of  the  cells,  cord-like  or  spherical  in  form  and  usually 
without  a  distinct  lumen,  lie  in  close  relation  to  the  capillary  blood-vessels 
that  ramify  between  them,  supported  by  the  delicate  connective  tissue  frame- 
work. Here  and  there,  however,  the  glandular  epithelium  surrounds  a 
lumen  which  may  contain  colloid  material,  and  thus  resemble  the  alveoli  of 
the  thyroid  body.  Such  colloid-containing  spaces  are  especially  numerous 
and  large  in  the  boundary  zone,  or  pars   intermedia,  between  the  anterior 


Chiei  cells. 


Chief  cell=; 


Capillarj' 


Colloid 


Capillary 


Chromophile  cells 


Fig.  357. — Section  of  anterior  lobe  of  pituitary  body,  showing  details  of  structure  ;  three  alveoli  contain 

Colloid  mateiial.     X  250. 

and  posterior  lobes.  Even  in  man,  but  to  a  very  much  more  marked  degree 
in  many  of  the  lower  mammals,  this  zone  contains  large  spaces  lined  with 
cuboidal  cells  and  more  or  less  filled  with  semifluid  material.  The  posterior 
wall  of  the  interlobar  space  consists  of  several  layers  of  cells  which  varyingly 
invade  the  adjoining  zone  of  the  posterior  lobule.  The  colloid  material  is 
to  be  regarded,  probably,  as  the  particular  secretion  of  the  glandular  segment 
of  the  hypophysis,  which,  moreover,  is  usually  conceded  a  place  among  the 
organs  of  internal  secretion. 

The  Posterior  Lobe. — The  posterior  and  smaller  division  of  the 
pituitary  bod)'-  is  directly  attached  to  the  floor  of  the  third  ventricle  by  means 
of  its  stalk  prolonged  from  the  infundibulum.  During  the  early  stages  of  its 
development,  this  lobe  is  represented  by  a  tubular  outgrowth,  whose  walls 
partake  of  the  general  character  of  the  parent  brain-vesicle.  In  man  the 
lumen  within  the  lower  end  of  the  diverticulum  later  entirely  disappeai's,  the 
posterior  lobe  being  solid.  In  some  lower  mammals,  notably  in  the  cat,  the 
lumen  is  retained  and,  in  rare  instances,  may  even  communicate  with  the 
interlobar  space.  In  the  adult  condition,  the  posterior  or  nervous  segment 
retains  few  histological  features  suggesting  its  cerebral  origin.     Of  the  demon- 


THE   MENINGES. 


3" 


strable  interlacing  fibres,  with  fusiform  enlargements  and  elongated  nuclei, 
none  can  be  identified  as  nerves,  while  of  the  numerous  cells  which  the  lobe 
contains,    only  a  few  of  large  size   and   pigmented   cytoplasm  uncertainly 


diencephalon 


Posterior  (cerebral)  lobe 


Anterior  (oral)  lobe 


Developing 

alveoli 


>lt 


-^■■'^iy-'^TiC'^^i: 


-Cartilage  of 
base  of  skull 


-Wail  of  oral  cavity 


Fig.  358. — Portion  of  sagittal  section  of  rabbit  embryo,  showing  developing  pituitary  body  ;  alveoli  are 
sprouting  from  wall  of  oral  diverticulum.     ,<  50. 

* 

resemble  ner\-ous  elements.  With  the  exception  of  neurogliar  cells,  the 
existence  of  definite  nervous  tissue  within  the  cerebral  lobe  of  the  mature 
human  hypophysis  is  very  doubtful. 

THE   MENINGES. 

The  entire  cerebro-spinal  axis  is  surrounded  by  three  membranes,  or 
meninges.  These  are:  ( i)  an  external  dense  fibrous  membrane,  the  dura 
7nater,  which  is  closely  attached  directly  to  the  inner  surface  of  the  skull  and 
in  the  vertebral  canal  forms  an  independent  loose  sheath  for  the  spinal  cord; 
(2)  an  internal  connective  tissue  tunic,  the  pia  mater ^  which  contains  the 
blood-vessels  supplying  the  nervous  tissue  and,  therefore,  is  adherent  to 
every  part  of  the  free  surface  of  the  brain  and  spinal  cord;  and  (3)  an  inter- 
mediate delicate  nonvascular  membrane,  the  arachtioid,  which  usually  lies 
close  to  the  dura  and  varies  in  its  relation  to  the  pia  mater. 

Between  the  dura  and  the  arachnoid  lies  a  narrow,  for  the  most  part 
capillary  cleft,  the  subdural  space,  which  contains  a  small  quantity  of  clear 
straw-colored  fluid,  of  the  nature  of  lymph.  The  arachnoid  and  the  pia 
mater  are  separated  by  a  much  larger  cavity,  the  subarachnoid  space,  which 
in  certain  places,  especially  on  the  basal  surface  of  the  brain,  reaches  exten- 
sive dimensions.  It  contains  the  cerebrospinal  fluid,  that  is  absolutely 
limpid  or  slightly  yellowish,  and  may  show  a  very  few  lymphocytes  (esti- 
mated normally  as  5  cells  per  cubic  millimeter  of  fluid).  The  cerebro-spinal 
fluid  is  produced  within  the  brain-ventricles,  from  the  tufts  of  blood-vessels 
of  the  several  choroid  plexuses,  and,  after  filling  the  ventricular  spaces  and 
the  central  canal  of  the  cord,  escapes  through  the  thin  roof  of  the  fourth 
ventricle  into  the  subarachnoid  spaces. 


312  NORMAL    HISTOLOGY. 

The  Dura  Mater. — Within  the  skull  {dura  mater  encephali),  the 
membrane  consists  of  an  outer  and  an  hmer  layer.  The  former  replaces  the 
periosteum  and  is  intimately  attached  to  the  bones  to  which  it  carries  nutritive 
blood-vessels  (branches  of  the  meningeal  arteries).  The  iimer  layer  forms 
the  incomplete  fibrous  partitions,  as  the  falx  and  tentorium,  which  separate 
and  support  the  several  subdivisions  of  the  brain.  Along  the  attachments 
of  these  partitions,  the  inner  layer  splits  to  form  the  walls  of  the  large  venous 
spaces,  the  dural  sinuses,  which  are  lined  with  endothelium  and  constitute 
the  channels  into  which  the  blood  returned  by  the  veins  from  the  nervous 
tissue  is  poured. 

Within  the  vertebral  canal  {dura  mater  spmali^),\he  dura  forms  a  loose 
sac  for  the  cord  which  corresponds  to  a  prolongation  of  the  inner  cranial 
layer.  It  loses  its  intimate  relation  to  the  bones  at  the  foramen  magnum  and 
lies  within  the  vertebral  canal  often  separated  from  the  periosteum  by  con- 
siderable tracts  of  areolar  tissue. 

In  structure,  the  dura  consists  of  closely  placed  bundles  of  unusually 
rigid  fibrous  tissue,  intermingled  with  elastic  fibres.  Although  the  latter 
exist  in  considerable  numbers,  especially  in  the  inner  layer  of  the  brain-dura, 
they  are  so  overshadowed  by  the  preponderance  of  the  fibrous  tissue  that  the 
membrane  as  a  whole  is  relatively  inelastic.  Within  the  outer  layer  in  the 
skull,  the  fibres  pursue  a  general  antero-lateral  to  postero-medial  direction, 
while  those  of  the  inner  layer  follow  an  opposite,  antero-medial  to  postero- 
lateral, course.  Within  the  spinal  sac,  their  disposition  is  chiefly  longitudinal. 
The  connective  tissue  cells  are  represented  by  flattened  plate-like  elements 
between  the  fibrous  bundles  and  some  plasma  cells  in  the  vicinity  of  the 
blood-vessels.  The  inner  surface  of  the  dura,  the  outer  wall  of  the  subdural 
space,  is  covered  by  a  continuous  layer  of  endothelial  cells.  The  existence 
of  isolated  patches  of  endothelium  on  the  outer  surface  of  the  membrane 
within  the  skull,  is  regarded  as  evidence  of  the  existence  of  Wmited' epidural 
spaces. 

The  blood-vessels  of  the  brain-dura,  not  taking  into  account  the  large 
venous  sinuses,  are  all  branches  from  the  various  meningeal  arteries.  In 
addition  to  supplying  the  dural  tissue,  their  purpose  is  to  provide  nourish- 
ment to  the  bones  of  the  cranium,  which,  therefore,  are  the  objective  dis- 
tribution for  the  larger  part  of  the  terminal  vessels.  The  outer  layer,  being 
virtually  the  periosteum,  contains  many  more  vessels  than  the  inner,  the  larger 
trunks  showing  as  elevated  ridges  on  the  cranial  surface.  Meningeal  veins 
are  also  present,  but,  in  many  cases,  do  not  accompany  the  arteries  and  pur- 
sue an  independent  course.  The  spinal  dura  contains  comparatively  few 
blood-vessels. 

The  nerves  within  the  dura  are  numerous  and  include  two  sets — those 
destined  for  the  walls  of  the  blood-vessels,  the  more  plentiful  and  sympa- 
thetic in  character,  and  the  less  numerous  nervi  proprii  which  contain  sen- 
sory fibres  derived  from  the  cranial  and  spinal  nerves.  They  end  in  free 
filaments  or  in  bulbus  expansions. 

The  Pia  Mater. — This  membrane,  the  vascular  tunic,  lies  in  contact 
with  all  parts  of  the  cerebro-spinal  axis  and,  since  it  contains  the  blood- 
vessels supplying  the  nervous  substance,  accurately  follows  all  the  irregu- 
larities of  the  surface  of  the  brain,  with  its  many  convolutions  and  fissures, 
and  of  the  spinal  cord.  Additionally,  in  certain  places  where  the  wall  of  the 
brain-tube  is  always  very  thin,  the  pia  pushes  before  it  the  attenuated  brain- 
layer  and  seemingly  gains  entrance  into  the  ventricles.  Examples  of  such 
invagination  are  afforded  in  the  relations  of  the  velum  interpositum  and  the 


THE   MENINGES. 


31; 


Vascular  tuft 


choroid  plexuses  to  the  lateral  and  third  ventricles  and  of  the  similar  plexuses 
in  the  roof  of  the  fourth  ventricle.  The  pia  mater  further  contributes  a 
sheath  to  each  nerve,  or  its  larger  component  bundles,  as  the  nerve  leaves 
the  brain  or  spinal  cord,  which  sheath  surrounds  the  nerve  as  it  crosses  the 
subarachnoid  space  and  for  a  variable  distance  beyond  its  emergence  from 
the  dural  sac. 

The  spinal  pia  consists  of  two  layers,  of  which  the  dense  outer  is 
composed  of  interlacing  stout  bundles  of  fibrous  tissue  mixed  with  elastic 
fibres  and  covered  externally  by  endothelium,  and  the  looser  inner  one  of 
less  closely  packed  fibro- elastic  strands.  These  layers  are  separated  here 
and  there  by  lymphatic  clefts  and  enclose  between  them  the  blood-vessels. 
The  latter  subdivide  within  the  pia  into  numerous  small  arteries  which, 
although  the  larger  trunks  frequently  anastomose,  enter  the  subjacent 
nervous  substance  as  "end-arteries,"  each  providing  the  entire  available 
blood-supply  for  a  definite  territory. 

The  brain-pia  consists  of  only  a  single  layer  which  corresponds  to  the 
inner  one  of  the  spinal  membrane  both  in  structure  and  relations.  The 
larger  vessels  lie  in  or  on  its  outer  part  and  in  certain  places  where  they  are 
of  large  diameter,  as  at  the  base 
of  the  brain,  project  within  the 
subarachnoid  space,  although 
covered  by  a  thin  en\elope  of 
pial  tissue.  As  the  vessels  pene- 
trate the  nervous  tissue,  they 
carry  with  them  a  sheath  of  pia 
mater,  at  first  loosely  but  later 
closely  applied.  These  consti- 
tute the  perivascular  lymph- 
sheaths  that  follow  the  arteries 
to  their  smallest  ramifications 
and  communicate,  through  the 
intrapial  lymph-clefts,  with  the 
subarachnoid  space.  Since  the 
arteries  entering  the  nervous  tis- 
sue, especially  the  cerebral  and 
cerebellar  cortex,  are  very 
abundant,  collectively  a  consid- 
erable amount  of  connective 
tissue  is  carried  with  the  vessels 
into  gray  matter,  the  larger  \'as- 
cular  septa  containing  fibro-elastic  tissue  as  well  as  neuroglia.  The  ultimate 
distribution  of  the  arteries  entering  the  spinal  cord  and  the  brain  is  described 
in  connection  with  those  organs  (pages  279  and  307).  In  certain  locations, 
particularly  the  base  of  the  brain  and  over  the  cervical  and  lumbar  enlarge- 
ments of  the  cord,  the  pia  sometimes,  especially  in  aged  subjects,  contains 
deeply  pigmented  branched  connective  tissue  cells. 

The  choroid  plexuses  of  the  ventricles  comprise  two  morphologically 
distinct  parts — the  vascular  pial  tissue  and  the  thin  covering  of  brain-wall. 
The  vascular  fringes  consist  of  numerous  capillary  convolutions,  the  cho- 
roidal glomeruli,  from  1-2  mm.  in  diameter,  embedded  in  the  pial  connec- 
tive tissue  stroma  and  covered  with  a  single  layer  of  cuboid  ependymal 
cells.  The  latter  contain  fat  and  pigment  particles  and  during  foetal  life 
bear  cilia. 


-  Velum 
interposituni 

Fig.  359. — Small  portion  of  injected  choroid  plexus  of 
lateral  ventricle;  surface  view.     X  25. 


314  NORMAL    HISTOLOGY. 

The  numerous  nerves  within  the  pia  mater  are  chiefly  sympathetic 
fibres  destined  for  the  walls  of  the  blood-vessels.  Within  the  skull  they  are 
derived  chiefly  from  the  plexuses  surrounding  the  internal  carotid  and  ver- 
tebral arteries;  within  the  spinal  pia  they  are  contributed  directly  from  the 
g-ray  sympathetic  rami.  Additional  nerve-fibres,  probably  sensory  in  func- 
tion, occur  in  small  numbers.  Their  mode  of  termination  is  uncertain, 
although  free  and  bulbus  endings  have  been  described. 

The  Arachnoid. — The  intermediate  membrane  is,  for  the  most  part, 
a  thin  connective  tissue  envelope  that  intervenes  between  the  dura  and  the 
pia  and,  notwithstanding  its  delicacy,  completely  separates  the  subdural 
from  the  subarachnoid  space.  It  contains  neither  blood-vessels,  lymphatics 
nor  nerves  and  consists  of  an  interlacement  of  flattened  bundles  of  fine 
fibrous  tissue  interspersed  with  elastic  fibres  and  plate-like  cells.  In  addi- 
tion to  the  main  sheet,  the  partition,  both  sides  of  which  are  covered  with 
erjdothelium,  numerous  trabeculae,  also  covered  with  endothelium,  extend 
across  the  subarachnoid  space  and  in  places  are  so  plentiful  as  to  convert 
the  cleft  into  a  sponge-like  structure.  In  contrast  to  the  pia  mater,  which 
closely  follows  the  surface  of  the  brain  and  cord,  the  arachnoid  is  separated 
from  the  cerebro-spinal  axis  and  its  immediate  covering  by  a  more  or  less 
extensive  space.  Over  the  convexities  of  the  convolutions,  however,  the 
arachnoid  and  the  pia  are  fused  into  a  single  membrane;  elsewhere  the  sub- 
arachnoid space,  filled  with  cerebrospinal  fluid,  is  considerable  and  on  the 
basal  surface  of  the  brain  very  extensive  and  represented  by  the  cisternce. 

Not  only  by  lymph-paths  along  the  nerve-trunks  and  larger  veins,  the 
cerebro-spinal  fluid  also  escapes  into  the  dural  sinuses  by  filtration  through 
local  tuft-like  accumulations  of  arachnoid  tissue,  situated  particularly  along 
the  superior  longitudinal  sinus.  These  tufts,  known  as  the  Pacchonian 
bodies,  consist  of  spongy  masses  of  arachnoid  tissue,  covered  externally 
with  endothelium,  which  push  before  them  the  greatly  attenuated  dura  and, 
overlaid  by  the  latter  and  the  endothelial  lining  of  the  blood-space,  project 
into  the  sinus  or  its  lateral  diverticula.  By  this  arrangement  the  cerebro- 
spinal fluid  that  occupies  the  interstices  of  the  arachnoid  tissue  filters  through 
the  interposed  structures  and  finds  its  way  into  the  venous  current  within  the 
dural  sinuses. 


THE  ORGANS   OF  SENSE. 

The  cells  directly  receiving-  the  stimuli  producina;-  the  sensory  impres- 
sions of  touch,  smell,  taste,  sight  and  hearing  are  all  derivations  of  the  ecto- 
derm— the  great  primary  sensory  layer  from  which  the  essential  parts  of  the 
organs  of  special  sense  are  differentiations.  The  olfactory  cells — nervous 
elements  that  correspond  to  ganglion-cells — retain  their  primary  relation, 
since  they  remain  embedded  within  the  invaginated  peripheral  epithelium 
lining  the  nasal  fossae,  sending  their  dendrites  towards  the  free  surface  and 
their  axones  into  the  -brain.  Usually,  however,  the  nerve-cells  connected 
with  the  special  sense-organs  abandon  their  superficial  position  and  lie  at 
some  distance  from  the  periphery,  receiving  the  stimuli  not  directly,  but 
from  the  epithelial  receptors  by  way  of  their  dendrites.  In  the  case  of  the 
most  highly  specialized  sense-organs,  the  eye  and  the  ear,  the  percipient 
cells  lie  enclosed  within  capsules  of  mesodermic  origin,  the  stimuli  reaching 
them  by  way  of  an  elaborate  path  of  conduction. 


THE  SKIN. 

Since  the  extensive  integumentary  sheet  that  clothes  the  exterior  of  the      | 
entire  body  not  only  serves  as  a  protective  investment,  an  efftcient  regulator 
of  body  temperature  and  an  important  excretory  structure,  but  also  contains 


—  Epidermis 

—  Papillary  stratum 

Reticular  stratum 


Hair  follicle 


Retinaculum 


Fat 


Fig.  360. — Section  of  skin,  showing  its  chief  layers — epidermis,  corium  and  tela  subcutanea.    X  17. 

the  special  end-organs  and  the  peripheral  terminations  of  the  sensory  nerves 
that  receive  and  convey  the  stimuli  producing  tactile  impressions,  the  skin 
may  be  appropriately  considered  along  with  the  other  sense-organs,  of  which 
it  may  be  regarded  as  the  primary  and  least  specialized.     On  the  other  hand, 

315 


3i6  NORMAL   HISTOLOGY. 

the  correspondence  of  its  structure  with  that  of  the  mucous  membranes,  with 
which  it  is  directly  continuous  at  the  orifices  on  the  exterior  of  the  body, 
emphasizes  the  close  relation  of  the  skin  to  the  alimentary  and  other  mucous 
tracts. 

This  general  investment,  the  tegmentum  commune,  includes  the 
skin  proper,  with  the  specialized  tactile  corpuscles,  and  its  appendages— X.\\& 
hairs,  the  nails  and  the  cutaneous  glands.  Its  average  superficial  area  is 
approximately  one  and  a  half  square  meters. 

The  skin  (cutis),  using  the  term  in  a  more  restricted  sense  as  applied 
to  the  covering  proper  without  its  appendages,  everywhere  consists  of  two 
distinct  portions,  a  superficial  epithelial  and  a  deeper  connective  tissue 
stratum,  which  are  derivatives  of  the  ectoderm  and  the  mesoderm  respec- 
tively. The  former,  the  epidennis,  is  devoid  of  blood-vessels,  the  capillary 
loops  never  reaching  farther  than  the  subjacent  coritcm,  as  the  outermost 
layer  of  the  connective  tissue  stratum  is  called.  The  thickness  of  the  skin, 
from  .  5-4  mm. ,  varies  greatly  in  different  parts  of  the  body,  being  least  on 
the  eyelids,  penis  and  nymphae,  and  greatest  on  the  palms  of  the  hands  and 

soles  of  the  feet  and  on  the  shoulders 
and  back  of  the  neck.  Of  the  entire 
thickness,  the  proportion  contributed 
by  the  epidermis  is  in  most  locaHties 
about  .  I  mm. 

The  Connective  Tissue  Stra- 

/         .  tum. — The  connective  tissue  stratum, 

^  ,  usually  much  the  thicker  portion  of  the 

skin,  includes  two  layers,  the  corium  and 
;  ;,  the  tela  subczitanca,   which,    however, 

'  are  so  blended  with  each  other  ^s  to  be 

',  ,  without  sharp  demarcation. 

The  corium  or  derma,  the  more 
I  superficial  and  compact  of  the  connec- 

tive tissue  layers,  lies  immediately  be- 
neath   the   epidermis  from  which  it  is 
'  always  well  defined.     With  the  excep- 

I.  /  tion  of  within  a  few  localities,  as  over 

iri--  the  forehead  and  the  external  ear,  the 

Fig!  36i.-Poriiou  of  corium  from  palmar  sur-    o^ter  Surface  of  the  corium  is  not  even 

face  of  hand  after  removal  of  epidermis;  each     but    bcSet    witll     elcvatioUS,     ridgeS,     Or 
range  includes  a  double  row  of  papillae,  which  .,,  1  •    1  1  j- 

underlie  the  surface  ridges  and  the  openings  of     papillae,    WhlCll    prodUCC    COrreSpOUOmg 

L^onTthemngesof^S^^^  modelling  of   the  opposcd  under  sur- 

face of  the  overlying  epidermis.  The 
best  developed  papillae  are  on  the  flexor  surfaces  of  the  hands  and  feet,  where 
they  attain  a  height  of  .  2  mm.  or  more  and  are  disposed  in  the  closely  set 
double  rows  that  underlie  the  cutaneous  ridges  on  the  palms  and  soles.  The 
patterns  formed  by  the  cutaneous  ridges  remain  throughout  life  unchanged 
and,  as  seen  in  imprints  of  the  fingers,  are  so  distinctive  for  each  individual 
that  they  afford  a  reliable  and  practical  means  of  identification.  The  mark- 
ings of  the  two  hands  are  symmetrical  and  sometimes  identical.  The  papillae 
afford  favorable  positions  for  the  lodgement  of  the  terminal  capillary  loops 
and  the  special  .organs  of  touch;  they  are  accordingly  grouped  as  vascular 
and  tactile. 

The  corium  is  subdivided  into  an  outer  papillary  stratum,  containing 
the  papillae,  and  a  deeper  retictdar  stratum,  composed  of  the  closely  inter- 


THE   SKIN.  317 

lacing  bundles  of  fibrous  and  elastic  tissue  that  are  continued  into  the  more 
robust  and  loosely  arranged  trabeculae  of  the  tela  subcutanea.  The  strata  of 
the  coriurn,  however,  are  so  blended  that  they  pass  insensibly  and  without 
definite  boundary  into  each  other.  Although  composed  of  the  same  histo- 
logical factors — bundles  of  fibrous  tissue,  elastic  fibres  and  connective  tissue 
cells — their  disposition  is  much  more  compact  in  the  dense  reticular  stratum 
than  in  the  papillary  layer.  While  the  general  course  of  the  fibrous  bundles 
within  the  corium  is  parallel  or  oblique  to  the  surface,  some  strands,  contin- 
ued upwards  from  the  underlying  subcutaneous  sheet, are  vertical  and  traverse 
the  stratum  reticulare  either  to  bend  over  and  join  the  horizontal  bundles,  or 
to  break  up  and  disappear  within  the  papillary  stratum.  The  elastic  tissue, 
which  constitutes  a  considerable  part  of  the  corium,  occurs  as  fibres  and  net- 
works. Within  the  reticular  stratum  these  form  robust  tracts  corresponding, 
with  the  general  arrangement  of  the  fibrous  bundles.  Towards  the  surface 
of  the  corium,  the  elastic  fibres  become  finer  and  more  branched  and  beneath 
the  epidermis  anastomose  to  form  the  delicate  but  close  subepithelial  elastic 
netzvork. 

The  tela  subcutanea,  the  deeper  layer  of  the  connective  tissue  por- 
tion of  the  skin,  varies  in  its  thickness  and  in  the  density  and  arrangement 
of  its  component  bundles  of  fibro-elastic  tissue,  with  the  amount  of  fat  and 
the  number  of  hair-follicles  and  glands  lodged  within  its  meshes.  The  latter 
are  irregularly  round  and  enclosed  by  tracts  of  fibrous  tissue,  some  of  which, 
known  as  the  vctinacula  cutis,  are  prolonged  from  the  corium  to  the  deepest 
parts  of  the  subcutaneous  stratum.  Here  they  often  blend  into  a  thin  but 
definite  sheet,  the  fascia  subcutanea,  which  forms  the  innermost  boundary 
of  the  skin  and  is  connected  with  the  subjacent  structures  by  strands  of 
areolar  tissue.  Where  such  loose  connection  is  wanting,  as  on  the  scalp, 
face,  palms  and  soles,  the  skin  is  intimately  bound  to  the  underlying  mus- 
cles or  fasciae  and  lacks  the  independent  mobility  that  it  elsewhere  enjoys. 
The  integument  covering  the  eyelids  and  penis  is  peculiar  in  retaining  to 
a  conspicuous  degree  its  mobility  although  devoid  of  fat.  Where  the  latter 
is  present  in  large  quantity,  the  t^xxw  panniculus  adiposus  is  often  applied  to 
the  tela  subcutanea. 

In  places  in  which  the  skin  glides  over  unyielding  structures,  the  inter- 
fascicular lymph-spaces  of  the  tela  subcutanea  may  undergo  enlargement 
and  fusion,  resulting  in  the  production  of  the  subcutaneous  mucous  bursce. 

In  addition  to  the  strands  of  iuvohmtajy  muscle  associated  with  the  hairs 
as  the  arrectores  pilorum,  unstriped  muscular  tissue  is  incorporated  with  the 
skin  in  the  mammary  areolae  and  over  the  scrotum  and  penis  (tunica  dartos). 
The  facial  muscles  having  largely  cutaneous  insertions,  the  skin  covering  the 
face  is  invaded  by  tracts  of  striated  musculav  tissue  that  penetrate  as  far  as 
the  corium. 

The  Epidermis. — The  epidermis  or  cuticle,  the  outer  portion  of  the 
skin,  consists  entirely  of  epithelium  and,  being  partly  horny,  affords  protec- 
tion to  the  underlying  corium  with  its  vessels  and  nerves.  The  thickness  of 
this  layer  varies  in  different  parts  of  the  body.  Commonly  from  .08- 
.  10  mm.,  it  is  greatest  on  the  flexor  surfaces  of  the  hands  and  feet,  where  it 
reaches  from  .5-.9  mm.  and  from  i.  1-1.3  mm.  respectively.  Where  exposed 
to  unusual  pressure,  as  on  the  palms  of  laborers  or  on  habitually  unshod 
soles,  the  epidermis  may  attain  a  thickness  of  4  mm. 

The  cuticle  consists  of  two  chief  layers,  the  deeper  stratum  germina- 
tivum,  containing  the  more  active  elements,  and  the  stratum  corneum,  the 
cells  of  which  undergo  cornification.      Between  these  layers  lies  a  third,  the 


318 


NORMAL    HISTOLOGY. 


stratum  mtennedium,  that  is  ordinarily  represented  by  only  a  single  row  of 
cells  to  which  the  name  stratum  granulosum  is  usually  applied.  This  layer 
marks  the  level  at  which  the  conversion  of  the  epithelial  elements  into  horny 
plates  begins  and  also  that  at  which  the  separation  effected  by  blistering 
usually  occurs.  On  the  palms  and  soles,  where  the  epidermis  attains  not  only 
great  thickness  but  also  higher  differentiation,  an  additional  layer,  the  stratum. 


Stratum  corneum 


Spiral  duct  of 
sweat-gland 


— -^>?V'   '  ;/  ' —  Stratum  lucidum 


stratum 
germinativum 


Corium 


Fig.  362. — Section  of  skin  from  sole  of  foot,  showinjr  layers  of  epidermis.     X  70. 

lucidum,  making  four  in  all,  may  be  recognized.  The  first  two  represent 
the  portion  of  the  epidermis  endowed  with  the  greatest  vitality  and  powers 
of  repair,  and  the  last  two  the  horny  and  harder  part. 

The  stratum  germinativum,  or  stratum  Malpighi,  rests  upon  the 
outer  surface  of  the  corium,  by  the  papillae  of  which  it  is  impressed  and, 
hence,  when  viewed  from  beneath  after  being  separated,  commonly  presents 
a  more  or  less  evident  network  of  ridges  and  enclosed  pits,  the  elevations 
corresponding  to  the  interpapillary  furrows  and  the  depressions  to  the 
papillae.  In  recognition  of  this  reticulation  the  name,  rete  Malpighi,  is 
sometimes  applied  to  the  deepest  layer  of  the  epidermis.  As  in  other  epi- 
thelia  of  the  stratified  squamous  type,  the  deepest  cells  are  columnar  and  lie 
with  their  long  axes  perpendicular  to  the  supporting  connective  tissue.  The 
basal  ends  of  the  columnar  cells  are  often  slightly  serrated  and  fit  into  cor- 


THE   SKIN. 


319 


responding  indentations  on  the  cerium.      Succeeding  the  single  row  of  col- 
umnar elements,  the  cells  of  the  stratum  germinativum  assume  a  pronounced 


Stratum  corneum 


Stratum  lucidum 


Stratum 


granulosum 


>*Jf^ stratum 

stjlS*  germinativum 


\ 


r ,      f    I 


Deepest  cells 

of  epidermis 


71/ Corium 

Fig.  365  —Portion  of  preceding  section,  showing  layers  of  epidermis  in  more  detail ;  only  the  deeper  part 
of  the  epidermis  is  represented.     X  2S0. 

polygonal  form,  but  become  somewhat  flatter  as  they  approach  the  stratum 
granulosum.      The  number  of  layers  included  in  the  germinal  stratum  is  not 
only  uncertain,  but  varies  with  the  relation  to  the  papillae,   being  greater 
between  than  over  these  pro- 
jections.     The  finelv  granular 
cytoplasm  of  the  cells  of  the 
stratum  germinativum  contains 
delicateyf^^r///^,  which  radiate 
from  the  nucleus  towards  the 
periphery.      The    librillse    are 
not    confined    to    the     cells, 
but  extend   beyond  and  pass 
across  the  intercellular  clefts  as 
delicate  protoplasmic  bridges 
;Fig.  364). 

The  stratum  granulo- 
sum is  exceptionally  well 
marked  on  the  palms  and  soles 
and  in  these  localities  includes 
from  two  to  four  rows  of  polygonal  cells,  that  stand  out  conspicuously  in 
stained  sections  by  reason  of  the  intensely  colored  particles  of  keratohyaliyi 
within  their  cytoplasm.  The  nature  of  this  peculiar  substance,  deposited 
within  the  body  of  the  cells  as  particles  of  irregular  form  and  size,  is  still 
uncertain.  It  is  probable  that  keratohyalin  is  in  some  way  derived  from 
disintegration  of  the  cytoplasm  and  represents  an  initial  stage  in  the  process 
ending  in  cornihcation  of  the  succeeding  layers  of  the  cuticle. 


Fibnllse 


Intercellular 
cleft 


Fig.  364. — Horizontal  section,  showing  intracellular  fibrilte 
within  cells  of  stratum  germinativum.    X  800. 


320 


NORMAL    HISTOLOGY. 


The  stratum  lucidum,  usually  wanting  in  other  localities,  in  the  palm 
and  sole  appears  as  a  thin,  almost  homogeneous  layer,  separating  the  cor- 
neous from  the  granular  layer.  With  the  latter  it  constitutes  the  stratum 
intermedium.  As  indicated  by  its  name,  the  stratum  lucidum  appears  clear 
and  without  distinct  cell  boundaries,  although  suggestions  of  these,  as  well 
as  of  the  nuclei  of  the  component  elements,  are  usually  distinguishable. 
The  cells  of  the  stratum  lucidum  contain  a  substance,  eleidin,  derived  from 
the  keratohyaline  particles,  which  soften  and  coalesce  into  a  homogeneous 
semifluid  material  that  fills  the  cells. 

The  stratum  corneum  includes  the  remainder  of  the  epidermis  and 
consists  of  many  layers  of  horny  epithelial  cells,  that  contain  pareleidin  and 
form  the  exterior  of  the  skin.  Where  no  stratum  lucidum  exists,  as  is  usually 
the  case,  the  corneous  layer  rests  upon  the  stratum  granulosum,  from  which 
its  horny  elements  are  being  continually  recruited.  During  their  migration 
towards  the  free  surface,  the  cells  lose  their  vitality  and  moisture  and  become 
more  flattened,  until  the  most  superficial  ones  are  converted  into  the  dead 
horny  scales  that  are  being  constantly  displaced  by  abrasion. 

The  pigmentation  of  the  skin,  which  e\'en  in  white  races  is  conspicuous 
in  certain  regions,  as  on  the  external  genital  organs  and  around  the  anus, 

__  depends  upon  the  pres- 

^_^^  ^T"^"^  "  ence   of    colored  parti- 

^    '    '-  '^  ^i^"^  cles.     These  lie  chiefly 

,^t  "  ^-'  within    the    epidermis, 

although,  when  the 
dark  hue  is  decided, 
a  few  small  branched 
])igmented  connective 
tissue  cells  may  appear 
within  the  subjacent  co- 
rium.  The  distribution 
of  the  pigment  particles 
varies  with  the  intensity 
of  color,  in  skins  of 
lighter  tints  being  prin- 
cipally limited  to  the 
columnar  cells  next  the 
corium.  With  increas- 
ing color  the  pigment 
particles  invade  the 
neighboring  layers  of 
epithelium  until,  in  the  dark  skin  of  the  negro,  they  are  found  within  the  cells 
of  the  stratum  corneum,  in  diminishing  numbers  towards  the  free  surface. 
Even  when  the  cells  are  dark  and  densely  packed,  the  colored  particles  never 
encroach  upon  the  nuclei,  which  appear  as  pigment-free  areas.  The  source 
of  the  pigment  within  the  epidermis  is  disputed,  some  assuming  a  transference 
of  the  colored  particles  by  means  of  wandering  cells  or  of  the  processes  of 
])igmented  connective  tissue  cells  that  penetrate  the  cuticle,  and  others 
accepting  an  independent  origin  in  situ  within  the  epithelial  elements.  While 
it  is  established  that  at  times  the  connective  tissue  cells  are  capable  of  modify- 
ing pigmentation,  it  is  equally  certain  that  the  earliest,  and  probably  also 
later,  intracellular  pigmentation  of  the  epidermis  appears  without  the  assist- 
ance of  the  connective  tissue  or  migratory  cells,  minute  colored  particles 
first  becoming  evident  within  the  epithelial  cytoplasm. 


■fj 

Pigmental 

^'«    ^^^&^ 

epidermis 

4^/ 

'W'!'c'\^ 

"~~-^  Papilla  of 
corium 

/ 

Duct  of 
sweat  gland 

1 

-  ^-^ 


,/ 


Fig.  365. — Section  of  skin  surrounding  anus,  showing  pigmentation 
deep  layers  of  the  epidermis.     X  50. 


THE   SKIN. 


321 


The  blood-vessels  of  the  skin  are  confined  to  the  connective  tissue 
portion  and  never  enter  the  cuticle.  The  arteries  are  derived  either  from  the 
trunks  of  the  subjacent  layer  as  special  cutaneous  branches  destined  for  the 
integument,  or  indirectly  from  muscular  vessels.  When  the  blood-supply  is 
generous,  as  in  the  palms  and  soles  and  other  regions  subjected  to  unusual 
pressure  or  exposure,  the  arteries  ascend  through  the  subdermal  layer  to  the 
deeper  surface  of  the  corium  where,  having  subdivided,  they  anastomose  to 
form  the  subcutaneous  plexus. 
From  the  latter  some  twigs  sink 
into  the  subdermal  layer  and  con- 
tribute the  capillary  networks  that 
supply  the  adipose  tissue  and  the 
sebaceous  glands.  Other  twigs, 
more  or  less  numerous,  pass  out- 
wards through  the  deeper  part  of 
the  corium  and  within  the  more 
superficial  stratum  unite  into  a 
second,  siibpapillary  plexus,  that 
extends  parallel  to  the  free  surface 
and  beneath  the  bases  of  the  pa- 
pillae. The  latter  are  supplied  by 
the  terminal  twigs  which  ascend 
vertically  from  the  subpapillary 
network  and  break  up  into  capil- 
lary loops  that  occupy  the  papillae 
and  lie  close  beneath  the  epidermis 
(Fig.  366).  The  arrangement  of 
the  cutaneous  veins,  more  complex 
than  that  of  the  arteries,  includes 
four  plexuses  lying  at  different 
levels  within  the  corium  and  ex- 
tending parallel  to  the  surfaces. 
The  first  and  most  superficial  one 
is  formed  by  the  union  of  the  radi- 
cles returning  the  blood  from  the 
papillae.  The  component  veins  lie 
below  and  parallel  to  the  rows  of 
papillae  and  immediately  beneath 
the  bases  of  the  latter.  At  a 
slightly  lower  level,  in  the  deeper 
part  of  the  stratum  papillare,  the 
venous  channels  proceeding  from 
the  subpapillary  network  join  to 
form  a  second  plexus.  A  third  occurs  about  the  middle  of  the  corium, 
while  the  fourth  shares  the  position  of  the  subcutaneous  arterial  plexus  at 
the  junction  of  the  corium  and  subdermal  strata.  The  deepest  plexus  receives 
many  of  the  radicles  returning  the  blood  from  the  fat  and  the  sweat-glands, 
the  remainder  being  tributary  to  the  veins  accompanying  the  larger  arteries 
as  they  traverse  the  tela  subcutanea. 

The  lymphatics  of  the  skin  are  well  represented  by  a  close  superficial 
plexus  within  the  papillary  stratum  of  the  corium  into  which  the  terminal 
lymph-radicles  of  the  papillae  empty.  The  relation  of  these  channels  to  the 
interfascicular  connective  tissue  spaces  is  one  only  of  indirect  communication, 


Fig.  366. — Section  of  injected  skin,  showing  general 
arrangement  of  the  blood-vessels  ;  the  terminal  loops 
occupy  the  papillae.    X  30. 


322  NORMAL    HISTOLOGY. 

since  the  lymphatics  are  provided  with  fairly  complete  endothelial  walls.  It 
is  probable  that  the  lymph-paths  within  the  papillae  are  closely  related  to  the 
intercellular  clefts  of  the  epidermis.  Migratory  leucocytes  often  find  their 
way  into  the  cuticle  where  they  then  appear  as  the  irregularly  stellate  cells  of 
Laiigerhans  seen  between  the  epithelial  elements.  A  wide-meshed  deep 
plexus  of  lymphatics  is  formed  within  the  subdermal  layer,  from  which  the 
larger  lymph-trunks  pass  along  with  the  subcutaneous  blood-vessels. 

The  numerous  nerves  within  the  highly  sensitive  integument  are  chiefly 
the  peripheral  processes  of  sensory  neurones  which  terminate  in  free  arbor- 
izations between  the  epithelial  elements  of  the  cuticle,  or  in  relation  with 
special  endings  located,  for  the  most  part,  within  the  corium  or  subdermal 
connective  tissue.  Some  sympathetic  fibres,  however,  are  present  to  supply 
the  tracts  of  involuntary  muscle  that  occur  within  the  walls  of  the  blood- 
vessels or  in  association  with  the  hairs  and  the  sweat-glands. 

On  entering  the  skin  the  medu Hated  nerves  traverse  the  subdermal 
layer,  to  which  they  give  off  twigs  in  their  ascent,  and,  passing  into  the 
corium,  within  the  papillary  stratum  divide  into  a  number  of  branches. 
Those  destined  for  the  epidermis  beneath  the  latter  break  up  into  many 
fibres  which,  losing  their  medullary  substance,  enter  the  cuticle  and  end  in 
ramifications  between  the  epithelial  cells  as  far  as  the  outer  limits  of  the 
stratum  germinativum.  The  ultimate  endings  of  the  fibrillae,  whether  taper- 
ing or  slightly  knotted,  always  occupy  the  intercellular  channels  and  are 
never  directly  connected  with  the  substance  of  the  epithelial  elements. 
Special  tactile  cells  (Fig.  no)  occur  in  the  human  epidermis,  particularly 
over  the  abdomen  and  the  thighs.  They  are  spherical  or  pyriform  and 
occupy  the  deeper  layers  of  the  cuticle  ;  on  the  side  directed  towards  the 
corium,  they  are  in  contact  with  the  end-plate  or  meniscus  of  the  nerve. 
The  nerve-fibres  particularly  concerned  with  the  sense  of  touch  terminate 
within  the  connective  tissue  portion  of  the  skin  in  special  end-organs. 
The  structure  of  these  end-organs  is  elsewhere  described  (pages  79-83), 
their  chief  locations  being  here  noted. 

Meissner' s  corpuscles  (Fig.  112)  are  especially  numerous  in  the  tactile 
cushions  on  the  flexor  surface  of  the  hands  and  feet.  While  much  more 
plentiful  in  all  the  tactile  pads  than  in  the  intervening  areas,  the  touch  cor- 
puscles are  most  abundant  in  those  on  the  volar  surface  of  the  distal 
phalanges,  where  they  approximate  twenty  to  the  square  millimeter  (Meiss- 
ner). Their  favorite  situation  is  the  apex  of  the  papillae,  where  they  appear 
as  elongated  elliptical  bodies,  sometimes  in  pairs,  whose  outer  pole  lies  imme- 
diately below  the  epidermis.  These  corpuscles  are  additionally,  although 
sparingly,  distributed  on  the  dorsum  of  the  hand,  the  flexor  surface  of  the 
forearm,  the  lips,  the  eyelids,  the  nipple  and  the  external  genital  organs. 

The  Vater- Pacinian  corpuscles  (Fig.  117)  are  well  represented  in  the 
hands  and  feet  and  usually  occupy  the  subdermal  tissue,  although  sometimes 
found  within  the  corium.  Their  distribution  corresponds  closely  to  that  of 
Meissner' s  corpuscles,  being  most  generous  beneath  the  tactile  cushions. 

The  Golgi-Mazzoni  co7p2cscles  are  modifications  of  the  Pacinian  bodies 
and,  like  the  latter,  are  found  within  the  subdermal  tissue. 

The  end-bulbs  of  Krause  (Fig.  113)  occur  within  the  corium,  either 
slightly  below  or  within  the  papillae,  on  the  lips  and  external  genital  organs, 
as  well  as  probably  in  other  regions. 

The  ge?iital  corpuscles  (Fig.  114)  lie  within  the  corium  of  the  modified 
skin  covering  the  glans  penis  and  the  prepuce  and  the  clitoris  and  surround- 
ing parts  of  the  nymphae. 


THE   HAIRS.  323 

The  ciid-organs  of  Ruffini  resemble  the  sensory  terminations  in  tendons 
(page  85)  and  he  within  the  deeper  parts  of  the  corium,  often  associated 
with  the  Pacinian  bodies. 

The  mode  of  ending  of  the  nerves  supplying  the  hairs  and  sweat-glands 
will  be  described  in  connection  with  those  structures  (pages  328  and  336). 

THE   HAIRS. 

The  appendages  of  the  skin — the  hairs,  nails,  and  cutaneous  glands—  are 
all  specializations  of  the  epidermis  ;  they  are,  therefore,  exclusively  of  ecto- 
dermic  origin. 

The  hairs  are  present  over  almost  the  entire  body,  the  few  localities  in 
which  they  are  absent  being  the  fle.xor  surface  of  the  hands  and  feet,  the 
extensor  aspect  of  the  terminal  segment  of  the  fingers  and  toes,  the  inner 
surface  of  the  prepuce  and  of  the  nymphae  and  the  glans  penis  and  clitoridis. 
With  the  exception  of  those  regions  in  which  the  growth  is  sufficiently  long 
to  constitute  a  complete  covering,  the  hairs  are  for  the  most  part  short  and 
scattered,  although  subject  to  great  individual  variation.  The  closest  set 
hairs  are  on  the  scalp,  on  the  top  of  the  head  numbering  from  300-320,  and 
in  the  occipital  and  frontal  regions  from  200-240  per  square  centimeter.  On 
the  chin  they  number  about  45,  on  the  mons  pubis  35,  on  the  extensor  sur- 
face of  the  forearm  24  and  on  the  back  of  the  hand  18  for  like  areas.  Even 
where  their  distribution  is  seemingly  uniform,  close  inspection  shows  the 
hairs  to  be  arranged  in  groups  of  from  two  to  five. 

In  their  thickness  the  hairs  show  much  variation,  not  only  in  different 
races,  individuals  and  regions,  but  also  in  the  same  person  and  part  of  the 
body,  as  on  the  scalp  where  fine  and  coarse  hairs  may  lie  side  by  side. 
The  thickest  scalp-hairs  have  a  diameter  of  162  //  and  the  finest  one  of  10  /x 
with  all  intermediate  sizes.  The  hairs  of  the  beard  vary  from  100-200  /j, 
and  those  on  the  pubes  from  50-135  p..  In  a  general  way,  hairs  of  light 
color  are  finer  than  dark  ones.  On  attaining  their  full  growth  without 
mutilation,  hairs  do  not  possess  a  uniform  thickness  throughout  their  length, 
since  they  diminish  not  only  towards  the  tip,  where  the  shaft  ends  in  a  point, 
but  also  towards  the  root.  This  feature  is  most  evident  in  short  hairs,  as  in 
those  of  the  eyebrows.  The  straight  and  curly  varieties  of  hair  depend 
chiefly  upon  differences  in  the  curvature  of  the  follicle  and  the  form  of  the 
hair.  In  the  case  of  straight  hairs  the  follicle  is  unbent  and  the  shaft  is 
cylindrical,  and  therefore  circular  in  cross-section;  hairs  that  are  wavy  or 
curly  spring  from  follicles  more  or  less  bent  and  are  flattened  or  grooved, 
with  corresponding  oval,  reniform,  or  irregularly  triangular  outlines  when 
transversely  cut. 

Each  hair  consists  of  two  parts,  the  shaft,  which  projects  beyond  the 
surface,  and  the  7'oot,  which  lies  embedded  obliquely  within  the  skin,  the 
deepest  part  of  the  root  expanding  into  a  club-shaped  thickening  known  as 
the  bulb.  The  root  is  covered  with  a  double  investment  of  epithelial  cells, 
the  inner  and  outer  root-sheaths,  which,  in  turn,  are  surrounded  by  a  con- 
nective tissue  envelope,  the  theea.  The  entire  sac-like  structure,  consisting 
of  the  hair-root  and  its  coverings,  constitutes  the  hair-follicle.  At  the 
bottom  of  the  latter,  immediately  beneath  the  bulb,  the  wall  of  the  follicle  is 
pushed  upwards  to  give  place  to  a  projection  of  connective  tissue,  the  hair- 
papilla,  which  carries  the  capillary  loops  into  close  relation  with  the  cells 
most  active  in  the  production  of  the  hair.  Save  in  the  case  of  the  finest 
hairs  (lanugo),  which  are  limited  to  the  corium,  the  hair-folhcles  traverse 


324 


NORMAL   HISTOLOGY. 


the  latter  and  end  at  varying  levels  within  the  fat-laden  subdermal  layer 
(panniculus  adiposus).  In  a  general  way  the  follicle  may  be  regarded  as  a 
narrow  tubular  invagination  of  the  epidermis,  at  the  bottom  of  which  the 


Shaft 


%7- Epidermis 


Sebaceous  gland 


Hair-papilla ^^^^ 


""f        Paniculus 
adiposus 


Fig.  367.— Section  of  scalp,  showing  longitudinally  cut  hair-follicles.    X  14. 

hair  is  implanted  and  from  the  entrance  of  which  the  shaft  projects.  The 
most  contracted  part  of  the  follicle,  the  7ieck,  lies  at  the  deeper  end  of  the 
relatively  wide  funnel-shaped  viouth  of  the  sac.  Closely  associated  with  the 
hair-follicle,  which  they  often  surround,  are  the  sebaceous  glands  that  pour 

their  oily  secretion  at  the  upper  third  of  the 
follicle  into  the  space  between  the  shaft  and 
the  wall  of  the  sac. 

The  Hair-Shaft. — In  many  thick  hairs, 
but  by  no  means  in  all,  three  parts  can  be  dis- 
tinguished— the  cuticle,  the  cortex,  and  the 
mediilla.  The  latter,  however,  is  usually  want- 
ing in  hairs  of  ordinary  diameter,  being  often 
also  absent  in  those  of  large  size. 

The  cuticle  of  the  hair  appears  as  a  trans- 
parent outermost  layer  marked  by  a  network 
of  fine  sinuous  lines,  the  irregular  meshes  of 
which  have  their  longest  diameter  placed 
obliquely  transverse.  These  lines  correspond 
to  the  free  borders  of  extremely  thin  glassy 
cuticle-plates  that  overlie  the  hair  as  tiles  on  a 
roof,  the  imbrication  involving  from  four  to  six 
layers.  The  cortical  substance,  often  constituting  practically  the  entire 
shaft,  consists  of  elongated  fusiform  cells  so  compactly  arranged  that  the  indi- 
vidual elements  are  only  distinguishable  after  the  action  of  disassociating 
reagents.     In  addition  to  the  remains  of  the  shrunken  nuclei,  the  hair- spindles, 


Fig.  368. — Portion  of  shaft  of  hair; 
h,  shaft  covered  with  cuticle ;  j,  corti- 
cal substance  exposed  by  removal  of 
cuticle;  »z,  medulla;  a,  i5i,  isolated  cells 
of  cuticle  and  cortical  substance  respec- 
tively.    X  240. 


THE   HAIRS.  325 

as  these  modified  epithelial  cells  are  called,  possess  fibrillae  that  pass  between 
adjacent  cells  similar  to  the  intercellular  bridges  in  the  epidermis.  A  variable 
amount  of  pigment,  either  diffuse,  or  as  granules  within  or  between  the 
spindles,  is  a  constant  constituent  of  the  cortical  substance.  In  blond  hair 
the  color  is  chiefly  diffuse,  the  pigment  granules  being  often  entirely  want- 
ing; in  hair  of  darker  shades,  the  granules  predominate  and  increase  in  in- 
tensity of  color  as  well  as  in  quantity.  As  the  hair  grows  outwards  from  the 
bulb,  it  loses  much  of  its  moisture,  and  in  consequence  later  contains 
minute  air-vesicles  that  replace  the  fluid  previously  occupying  the  clefts 
between  the  hair-spindles. 

The  medulla,  when  well  represented,  is  seen  as  an  axial  stripe,  some- 
what uneven  in  outline,  that  varies  with  illumination,  with  transmitted  light 
appearing  as  a  dark  band  and  with  reflected  light  as  a  light  one.  This 
peculiarity  depends  upon  the  presence  of  air  imprisoned  between  the 
shrunken  and  irregular  medullary  cells — dried  and  cornified  epithelial 
elements  which  are  connected  by  branching  processes  into  a  network  in- 
completely filling  the  medulla.  The  air  within  the  shaft  modifies  the  color 
of  the  hair,  since  the  resulting  reflex  tends  to  lessen  the  intensity  of  the  tint 
directly  referable  to  the  pigment.  Even  when  conspicuous,  the  medulla 
does  not  extend  the  entire  length  of  the  hair,  often  being  interrupted  and 
always  disappearing  before  reaching  the  tip. 

The  Hair-Follicle. — This  structure  includes:  (i)  a  connective  tissue 
sheath,  the  theca,  contributed  by  the  corium;   (2)  an  epithelial  lining,  the 

Outer  root  sheath    ^t  j  jv 

_.  Hair  surrounded  by 

t^^^  ?v        .y^  '^  ^  inner  root  bheath 


f^i 


\       / 


Adipose  tissue 


f 


FiDrous  ussue         Jk^^-V  i^^°~^ 
Fig.  369. — Horizontal  section  of  scalp,  showing  group  of  transversely  cut  hair-follicles.     X  65. 

outer  root-sheath,  continued  from  the  deepest  layer  of  the  epidermis;  and 
(3)  the  inner  root-sheath,  an  epithelial  investment  probably  differentiated 
within  the  follicle,  and  not  a  direct  prolongation  from  the  cuticle. 

The  theca  foUiculi  includes  three  strata:  an  outer,  composed  of 
loosely  disposed  longitudinal  bundles  of  fibrous  tissue  with  a  few  cells  and 
elastic  fibres;  a  middle  one,  made  up  of  closely  placed  circular  bundles;  and 
a  very  thin,  homogeneous  inner  coat,  the  glassy  membrane,  which  repre- 
sents an  unusually  well  developed  basement  membrane  separating  corium 


326 


NORMAL    HISTOLOGY. 


from  cuticle.  Greatly  attenuated,  it  is  prolonged  over  the  hair-papilla, 
which,  as  a  special  vascularized  thickening  of  the  connective  tissue  of  the 
follicle,  carries  nutrition  to  the  bulb  of  the  growing  hair. 

The  outer  root-sheath  is  the  continuation  of  the  stratum  germinativum 
alone,  the  other  layers  of  the  epidermis  thinning  out  and  disappearing  before 
reaching  the  neck  of  the  follicle.  Its  cells  present  the  characteristics  of  those 
of  the  germinating  layer,  with  exceptionally  well  marked  fibrillce.  On  ap- 
proaching the  level  of  the  papilla,  the  outer  root-sheath,  which  farther  above 
consists  of  numerous  layers,  rapidly  diminishes  in  thickness  until,  on  the 
sides  of  the  papilla,  it  is  reduced  to  a  single  row  of  columnar  cells. 

The  inner  root-sheath,  which  is  best  developed  over  the  middle  third 
of  the  hair-root  and  fades  away  on  reaching  the  upper  third,  includes  three 


Theca  folliculi 


Henle's  layer 

of  inner  root-sheath 


Outer  root-sheath 


Fig.  370.— Hair-follicle  cut  across  about  the  middle,  showing  hair  surrounded  by  the  root-sheaths.    X  285. 

layers.  The  outer,  known  as  Henle's  layer,  consists  of  a  single  row  of  flat 
polygonal  cells,  often  partially  separated  by  oval  spaces.  Their  nuclei  are 
very  indistinct  or  invisible  within  the  cornified  cytoplasm.  The  middle  or 
Huxley  s  layei%  also  horny  in  nature,  often  comprises  only  one  stratum  of 
nucleated  cuboidal  cells,  but  in  the  thicker  hairs  two  or  even  three  rows  of 
irregularly  interlocked  cells  may  be  present.  The  third  layer,  known  as  the 
sheath-cuticle,  resembles  the  external  coat  of  the  hair,  against  which  it  lies, 
in  being  extremely  thin  and  composed  of  fiat  horny  plates.  The  latter, 
however,  are  always  nucleated  and  so  disposed  that  they  are  opposed  to  the 
serrations  of  the  thicker  hair-cuticle. 

Traced  towards  the  bottom  of  the  follicle,  the  root-sheaths  and  the  hair, 
which  above  are  sharply  defined  from  one  another,  become  more  and  more 
alike  until,  in  the  immediate  vicinity  of  the  hair-papilla,  they  blend  into  a 
still  imperfectly  differentiated  mass  of  cells.      The  deepest  elements  of  this 


THE    HAIRS. 


327 


complex,  however,  are  cuboidal  t)r  low  columnar  and  form  an  uninterrupted 
tract  over  the  papilla,  continuous  with  the  outermost  cells  of  the  outer  root- 
sheath.  It  is  from  the  proliferation  of  these  deepest  cells  that  the  formative 
material,    or  viatrix,  is  provided   to  meet  the  requirements  of  growth  and 


Cells  forming 
ineilulla 


Cells  forming 
cortex 


Blood-vessel 


Fig.  371.— Longitudinal  section  through  deepest  part  of  hair-follicle.    X  285. 

replacement  of  the  hairs.  Of  the  three  parts  of  the  hair,  the  medulla  is 
produced  by  the  cells  overlying  the  summit  of  the  papilla,  while  those  con- 
verted into  the  cortical  substance,  cuticle  and  inner  root-sheath  occupy  the 
sides  of  the  papilla  and  deepest  part  of  the  follicle. 

With  few  exceptions,  the  hair  follicles  are  associated  with  two  or  more 
sebaceous  glands,  rarely  with  only  one,  the  ducts  of  which  open  into  the 


328 


NORMAL   HISTOLOGY. 


sac  in  the  vicinity  of  the  neck.  The  glands  usually  lie  on  the  side  towards 
which  the  hair  inclines,  but  sometimes,  especially  in  the  case  of  the  smaller 
hairs,  they  may  completely  surround  the  follicle.  Since  these  glands  are 
outgrowths  from  the  same  tissue  that  lines  the  follicles,  their  ducts  pierce  the 
outer  root-sheath,  bringing  their  oily  secretion  into  direct  relation  with 
the  hairs. 

Most  of  the  larger  hair-follicles,  particularly  those  of  the  scalp,  are 
provided  with  ribbon-like  bundles  of  involuntary  muscle,  called  the  arrec- 
tores  pilorum  in  recognition  of  their  effect  on  the  hairs.  They  arise  from 
the  superficial  part  of  the  corium,  pass  obliquely  downwards  to  be  inserted 
into  the  sheath  of  the  hair-follicle  near  the  junction  of  corium  and  subdermal 
tissue,  and  on  the  side  corresponding  with  the  inclination  of  the  hair  and  the 
situation  of  the  sebaceous  glands.  Since  the  latter  are  closely  embraced  by 
the  muscular  bands,   contraction  of  the  muscles  exerts  pressure  upon  the 

glands  and  facilitates  the  dis- 
l  ^  '''C>  charge  of  their  secretion,  the 

^^^^  '-^     -"         sebum. 

The  blood-vessels  sup- 
plying the  hair-follicle,  which 
in  a  sense  constitute  a  special 
system  'for  each  sac,  include 
the  capillary  loops  ascending 
within  the  hair-papilla  and 
the  network  of  capillaries  sur- 
rounding the  follicle  immedi- 
^  jT    r ^     ^r\\\cK  I       \     '^^^       ately  outside  the  glassy  mem- 

,Vr      /      ^ yJ.     7       ',    Vv       a  t  V\     brane.      The  first  are  derived 

\b>T    '  '  ^<:-'\.   1  '    ''^    ^^-^\  V        ^''     from  a  small  special  twig  that 

ascends  to  the  follicle,  and  the 
second  from  the  subpapillary 
network  of  the  corium.  With 
the  exception  of  those  draining 
the  papilla,  which  are  tributary 
to  the  deeper  stems,  the  veins 
join  the  subpapillary  plexus. 

The  nerves  distributed 
to  the  follicles  follow  a  fairly 
definite  arrangement.  Usually  each  hair-sac  is  supplied  by  a  single  fibre, 
sometimes  by  two  or  more,  which  approaches  the  follicle  immediately  below 
the  level  of  the  mouth  of  the  sebaceous  glands.  After  penetrating  the  fibrous 
sheath  as  far  as  the  glassy  membrane,  the  nerve-fibre  separates  into  two 
divisions  that  encircle  more  or  less  completely  the  follicle  and  on  the  opposite 
side  break  up  into  terminal  arborizations.  The  nerve-endings  usually  lie 
on  the  outer  surface  of  the  glassy  membrane  within  the  middle  third  of  the 
follicle  and  only  exceptionally  are  found  within  the  outer  root-sheath  or  the 
hair-papilla. 

Development. — The  primary  development  of  the  hair  begins,  about 
the  end  of  the  third  month  of  foetal  life,  as  localized  proliferations  of  the 
epidermis.  In  section  these  appear  as  lenticular  thickenings  and  on  the 
surface  as  slight  projections.  Very  soon  solid  epithelial  cylinders  sprout  from 
the  deeper  surface  of  these  areas  and  invade  the  subjacent  corium  to  form  the 
rudiments  of  the  hair-follicles.  The  original  uniform  outline  of  these  proc- 
esses is   early  replaced  by  a  flask-shaped  contour  in   consequence  of  the 


Papillary 
twig 


Fig.  372. — Section  of  injected  scalp,  showing  capillary  net- 
works surrounding  hair- toUicles  and  twigs  entering  papillae. 
X20. 


DEVELOPMENT   OF   THE   HAIRS. 


329 


enlargement  of  their  ends,  which  in  their  growth  surround  connectixe  tissue 
processes  to  form  the  hair-papilla:.  The  embryonal  connective  tissue  imme- 
diately surrounding  the  epidermal  ingrowth  differentiates  into  the  fibrous 
sheath  and  the  glassy  membrane. 

Meanwhile  and  even  before  the  formation  of  the  papilla,  the  epithelial 
contents  of  the  young  follicle  differentiate  into  an  axial  strand  of   spindle 
cells,  that  later  undergoes  keratinization  and  becomes  the  hair-shaft,  which 
^  grows  by  subsequent  additions  from  the  ma- 

trix surrounding  the  papilla.  In  addition  to 
forming  the  outer  root-sheath,  the  peripheral 
elements  contribute  the  matrix-cells  that 
occupy  the  fundus  of  the  follicle  and  surround 
the  papilla.  The  cells  covering  the  summit 
and  adjacent  sides  of  the  papilla  are  converted 
into  elongated  spindles  that  gradually  become 
horny  and  assume  the  characteristics  of  the 
cortical  substance  of  the  hair.     When  present, 


Fig.  373. — Sections  of  developing 
skin  showing  earliest  stages  in  forma- 
tion of  hair-foHicle ;  in  D  epithelial 
cylinder  is  invading  mesoderm.    >^  90. 


follicle 


Fig.  374. — Developing  skin,  showing  later  stages  of  hair- 
follicles  ;  mesoderm  is  forming  hair-papilla  and  fibrous  sheath 
of  follicle.     X  90. 


the  medulla  is  developed  by  the  transformation  of  the  cells  occupying  the  sum- 
mit of  the  papilla,  which  enlarge,  become  less  granular  and  grow  upwards  as  an 
a.xial  strand  that  invades  the  chief  substance  of  the  hair  and  accumulates 
keratohyalin  within  its  cells.  The  pigment  particles,  which  appear  later, 
are  first  evident  in  the  hair-bulb  and  probably  arise  within  the  epithelial  tissue. 
The  elements  of  the  hair-cuticle  and  of  the  inner  root-sheath  are  differentiated 
from  the  matrix-cells  at  the  sides  of  the  papilla.  The  tall  columnar  elements 
become  elongated  and  converted  into  the  cornified  plates  of  the  cuticle  both 
of  the  hair  and  of  the  inner  root-sheath.  The  layers  of  Huxley  and  of 
Henle  are  derived  from  cells  that  soon  exhibit  granules  of  keratohyalin,  so 
that  on  reaching  the  level  of  the  summit  of  the  papilla  the  process  of  corni- 
fication  has  been  established. 

The  growth  of  the  hair  takes  place  exclusively  at  the  lower  end  of 
its  bulb,  where,  so  long  as  the  hair  grows,  the  conversion  of  the  matrix-cells 
into  the  substance  of  the  hair  is  continuously  progressing.  By  this  process 
the  substance  already  differentiated  is  pushed  upwards  by  the  cells  undergoing 
transformation  and  these,  in  turn,  are  displaced  by  the  succeeding  elements. 
In  this  way,  by  the  addition  of  new  increments  in  its  bulb,  the  hair  is  forced 
onwards  and,  in  the  case  of  those  first  formed,  through  the  epidermis  that 
still  blocks  the  mouth  of  the  follicle.  This  eruption  begins  on  the  scalp  and 
regions  of  the  eyebrows  about  the  fifth  foetal  month  and  on  the  extremities 
about  a  month  later. 


330 


NORMAL   HISTOLOGY. 


The  hairs  covering  the  foetus  are  soon  shed,  during  the  last  weeks  of  gestation 
and  immediately  following  birth,  and  are  replaced  by  the  stronger  hairs  of  childhood. 
These  latter,  too,  are  continually  falling  out  and  being  renewed  until  puberty,  when  in 

many  localities,  as  on  the  scalp, 
face,  axillae  and  external  genital 
organs,  they  are  gradually  replaced 
by  the  much  longer  and  thicker 
hairs  that  mark  the  advent  of  sexual 
maturity.  Even  after  attaining  their 
mature  growth,  the  individual  life 
of  the  hairs  is  limited,  those  on 
the  scalp  probably  retaining  their 
vitality  for  from  two  to  four  ^-ears 
and  the  eyelashes  for  only  a  few 
months.  The  change  of  hair,  that 
is  continually  and  insensibly  occur- 
ring in  man,  includes  the  atrophy 
of  the  old  hair  and  the  development 
of  the  new  one. 

The  earliest  manifestations  of 
this  atrophy  are  reduction  in  the 
size  and  differentiation  of  the  mass 
of  matrix-cells  at  the  bottom  of 
the  follicle  and  the  diminution  of 
the  hair-papilla.  The  progressive 
reduction  of  the  matrix  is  accom- 
panied by  the  production  of  a  club-shaped  enlargement  of  the  hair,  between  which  and 
the  shrunken  matrix  a  strand  of  atrophic  epithelial  cells  for  a  time  remains.  With 
the  continued  progress  of  these  changes,  the  root  of  the  club-hair,  as  the  degenerating 
hair  is  termed,  shortens  so  that  the  bulbus  enlargement  recedes  from  the  bottom  of 
the  hair-sac  until  it  lies  just  below  the  narrow  neck  of  the  follicle,  where  it  remains 
for  a  longer  or  shorter  period  until  the  hair  is  dislodged  and  finally  discarded.  While 
the  old  hair  is  still  lodged  in  the  upper  part  of  the  follicle,  the  first  steps  towards  its 
replacement  are  initiated  by  the  stratum  germinativum  of  the  old  hair-sac,  the  deepest 
follicie-cells  contributing  by  proliferation  the  material  from  which  the  new  hair  is 
developed  in  a  manner  essentially  the  same  as  that  by  which  its  predecessor  Vv'as  formed. 


Sebaceous  gland 


Hair 


Root-sheath 


Bulb 


Papilla 


Fig.  375. — Later  stage  of  developing  follicle  ;  the  hair  is  now 
difEerentiated.     X  80. 


THE   NAILS. 

The  nails,  the  horny  plates  overlying  the  ends  of  the  dorsal  surfaces  of 
the  fingers  and  toes,  correspond  to  the  claws  and  hoofs  of  other  animals  and, 
like  them,  are  composed  exclusively  of  epithelial  tissue.  They  are  special- 
izations of  the  epidermis  and,  therefore,  may  be  removed  with  the  cuticle 
without  mutilation.  The  entire  nail-plate  is  divided  into  the  body,  which 
includes  the  exposed  portion,  and  the  root,  which  is  embedded  beneath  the 
skin  in  a  pocket-like  recess,  the  nail-groove.  The  modified  skin  supporting 
the  nail-plate,  both  the  body  and  the  root,  constitutes  the  nail-bed,  the 
cutaneous  fold  overlying  the  nail  being  the  nail-wall.  During  life  the  nail 
shows  color-zones,  its  projecting  portion  being  immediately  followed  by  a  very 
narrow  yellow  band  that  corresponds  to  the  line  along  which  the  stratum 
corneum  of  the  underlying  skin  meets  the  under  surface  of  the  nail-plate. 
The  succeeding  and  larger  part  of  the  nail  is  occupied  by  the  broad  pink 
zone  which  owes  its  rosy  tint  to  the  blending  of  the  color  of  the  blood  in  the 
underlying  capillaries  with  that  of  the  horny  substance.  On  the  thumb 
constantly,  but  on  the  fingers  often  only  after  retraction  of  the  cuticle,  is  seen 
a  transversely  oval  white  area,  the  bmula,  which  marks  the  position  of  the 
underlying  matrix. 


THE   NAILS. 


331 


The  substance  of  the  nail-plate  consists  entirely  of  flattened  horny- 
epithelial  cells,  very  firmly  united  and  containing  the  remains  of  their 
shrunken  nuclei;  hence  it  is  also  called  stratum 
corncion  unguis.  These  cornified  scales  are 
disposed  in  lamellae,  which,  in  transverse  sec- 
tion, pursue  a  course  in  general  parallel  with 
the  dorsal  surface.  In  nails  which  possess  the 
longitudinal  ridges,  however,  the  latter  coincide 
with  an  upward  arching  of  the  lamellae  dependent 
upon  the  conformation  of  the  nail-matrix.  In 
longitudinal  section  the  lamellation  is  oblique, 
extending  from  above  downwards  and  forwards. 
Minute  air-\-esicles,  imprisoned  between  the  horny 
scales,  are  constant  within  the  nail-substance. 
When  these  occur  in  unusual  quantities,  they 
give  rise  to  white  spots  in  the  nail. 

The  nail-bed  is  divided  into  a  proximal, 
a  middle  and  a  distal  region,  each  of  which 
exhibits  structural  peculiarities  and  corresponds 
respectively  to  the  white,  rosy  or  yellow  zone 
seen  from  the  dorsal  surface  of  the  nail.  The 
most  important  of  these  regions  is  the  proximal, 
known  as  the  matrix,  which  lies  beneath  the 
white  area  and  alone  is  concerned  in  the  produc- 
tion of  the  nail.  So  long  as  the  matrix  is  healthy,  it  is  capable  of  replacing 
even  an  entire  lost  nail  bv  a  new  one. 


Fig.  376. — Part  of  finger,  showing- 
relations  of  tlie  nail ;  rt,  i,  distal 
and  proximal  borders  of  nail ;  c, 
nail-wall ;  <f,  line  along  which  epi- 
dermis passes  to  under  surface  of 
nail-plate;  c,  lunula. 


The  corinm  of  the  nail-bed  varies  in  the  different  regions  in  the  arrangement  and 
size  of  its  elevations.     Within  the  proximal  third  of  the  matrix,  these  elevations  occur 


Nail-plate' 
Epidermis- 


and 


Corium 
of  nail-bed ' 


Subcutineous  tissue 
Stratum  ^trminitnum 
Sti  itum  corneum 

Eponjchium 


^  Transformation 
zone 


Y 


577. — Longitudinal  section  of  proximal  part  of  nail  lying  within  the  nail-groove.     X  3°- 


as  low  papillae,  which  decrease  in  height  and  number  until  they  disappear,  an  even 
field  occupying  the  middle  of  the  matrix.     This  field  is  succeeded  by  one  possessing 


332 


NORMAL    HISTOLOGY. 


closely  set  low  narrow  longitudinal  ridges,  that  at  the  distal  margin  of  the  lunula 
suddenly  give  place  to  more  pronounced,  but  less  numerous  broader  lineal  elevations. 
These  continue  as  far  as  the  distal  end  of  the  nail-bed  and  are  then  replaced  by  papillae. 
Owing  to  strong  fibrous  bands  and  the  absence  of  the  usual  layer  of  fatty  subdermal 
tissue,  the  corium  of  the  nail-bed  is  closely  attached  to  the  bone. 

The  epidermis  jmderlying  the  nail  is  of  especial  interest  in  view  of  its  genetic 
activity.  While  the  stratum  germinativum  of  the  skin  covering  the  finger-tip  passes 
directly  and  insensibly  onto  the  nail-bed,  the  entire  extent  of  which  it  invests  {stratum 
germinativum  utiguis),  the  stratum  corneum  ends  on  reaching  the  under  surface  of 
the  nail-plate,  the  line  of  apposition  corresponding  to  the  narrow  yellow  zone  which 


Nail-bed 


Nail-Dlate 


Stratum  corneum 

and 

Stratum 

germinativum 

of  nail-wall 


Eponychium 
Margin  of  nail 


Corium 


Fig.  378.— Section  across  nail-wall  and  adjoining  part  of  nail-plate  and  nail-bed.    X  90. 


defines  the  distal  boundary  of  the  rosy  area.  Beneath  the  latter,  therefore,  the  epi- 
dermis of  the  nail-bed  consists  of  the  stratum  germinativum  alone,  which,  without 
cornification  of  any  of  its  cells,  rests  against  the  under  surface  of  the  nail.  Beneath 
the  white  zone,  that  is,  within  the  matrix,  the  epidermis  includes  a  half-dozen  or  more 
layers  of  the  usual  elements  of  the  stratum  germinativum,  surmounted  by  a  like 
number  of  strata  of  cells  distinguished  by  a  peculiar  brownish  color.  On  reaching 
the  nail  these  modified  epithelial  elements  pass  into  the  substance  of  the  plate,  the 
constituent  cells  of  which  they  directly  become.  Since  the  transformation  of  the  cells 
of  the  stratum  germinativum  into  those  of  the  nail-plate  is  confined  to  the  matrix, 
it  is  evident  that  the  continuous  growth  of  the  nail  takes  place  along  the  floor  and 
bottom  of  the  nail-groove,  the  last  formed  increment  of  nail-substance  pushing 
forwards  the  previously  differentiated  material  and  thus  forcing  the  nail  towards  the 
end  of  the  digit.  As  the  nail  leaves  the  groove,  a  part  of  the  stratum  germinativum 
of  the  nail-wall  blends  with  the  epidermis  and  is  prolonged  for  a  variable  distance  over 
the  dorsal  surface  of  the  nail-plate  as  a  delicate  membranous  sheet,  the  eponychium, 
which  usually  ends  in  a  ragged  and  abraded  border. 

THE   CUTANEOUS  GLANDS: 
These  structures  include  two  chief  varieties,  the  sebaceous  and  the  sweat- 
glands,  together  with  certain  modifications,  as  the  ceruminous  glands  within 
the  external  auditory  canal,  the  circumanal  glands,  the  tarsal  and  ciliary 


THE   CUTANEOUS   GLANDS. 


333 


glands  within  the  eyelid  and  tlie  mammary  glands.  In  all,  the  epithelial 
tissues — the  secreting  elements  and  the  lining  of  the  ducts — are  derivatives  of 
the  ectoderm  and,  therefore,  genetically  related  to  the  epidermis. 

The  Sebaceous  Glands. — Although  these  structures,  the  glandulcc 
sebaceae,  arechieliy  associated  with  the  hair-follicles,  they  also  occur,  although 


\ 


Mouth 
ot  giand 


Duct 


Alveoli 


Corium 


m^ 


Fig.  379. — Sebaceous  glands  in  skin  covering  ala  of  nose.    X  60. 

less  frequently,  independently  and  in  those  parts  of  the  skin  in  which  the 
hairs  are  wanting,  as  on  the  lips,  prepuce,  and  labia  minora.     The  size  of 
these  glands  bears  no  relation  to  that  of  the  hairs,  since  among  the  smallest 
(.2-4  mm.)  are  those  on  the  scalp.     The  largest 
(.5-2.  mm. )  are  found  on  the  mons  pubis,  scrotum,  r--7^}\ 

external  ear  and  nose.  Conspicuous  aggregations, 
modified  in  form,  occur  in  the  eyelids  as  the  Mei- 
bomian glands. 

The  smallest  sebaceous  glands  are  each  little 
more  than  tubular  diverticula,  dilated  at  the  closed 
ends.  In  those  of  the  larger  size,  the  relatively 
short  duct  subdivides  into  several  expanded  com- 
partments, which,  in  the  largest  glands,  may  be 
replaced  by  groups  of  irregular  alveoli,  with  uncer- 
tain ducts  that  converge  into  a  short  wide  common 
excretory  passage. 

The  structural  components  of  these  glands 
include  ■st.  fibrous  envelope,  ■st.  me mbrana  propria,  and  the  epithelium,  the  first 
two  being  continuous  with  the  corresponding  coverings  of  the  hair-follicle. 
The  epithelium  continued  into  the  ducts  and  alveoli  of  the  sebaceous  glands 
is  directly  prolonged  from  the  outer  root-sheath  of  the  epidermis,  where 
associated  with  the    hair-follicles,   or   from  the  epidermis   where  the    hairs 


Fig.  3S0. — Cells  from  alveoli 
of  sebaceous  glands,  conspicu- 
ously showing  reticulated  cyto- 
plasm.    X  650. 


334  NORMAL    HISTOLOGY. 

are  wanting.  The  periphery  of  the  alveolus  is  occupied  by  a  single,  or 
incompletely  double,  layer  of  flattened  and  imperfectly  defined  basal  cells. 
These  rest  immediately  upon  the  membrana  propria  and  are  distinguished 
by  their  dark  cytoplasm  and  outwardly  displaced  oval  nuclei.  Passing 
towards  the  centre  of  the  alveolus,  the  next  cells  contain  a  number  of 
small  oil  drops  which,  with  each  successive  row  of  cells,  become  larger 
and  appropriate  more  and  more  space  at  the  expense  of  the  protoplasmic 
reticulum  in  which  they  are  lodged.  In  consequence,  the  cells  occupy- 
ing the  centre  of  the  alveoli,  which  are  completely  filled  and  without  a 
lumen,  contain  little  more  than  fat.  As  the  cells  are  escaping  from  the 
glands  they  lose  their  nuclei  and  individual  outlines  and,  finally,  are  merged 
as  debris  into  the  secretion,  or  sebimt,  with  which  the  hairs  and  skin 
are  anointed.  The  necessity  for  new  cells,  created  by  the  continual 
destruction  of  the  glandular  elements  that  attends  the  activity  of  the  seba- 
ceous glands,  is  met  by  the  elements  recruited  from  the  proliferating  basal 
cells,  which  in  turn  pass  towards  the  centre  of  the  alveolus  and  so  displace 
the  accumulating  secretion. 

The  Sweat-Glands. — These  structures,  the  glandid<z  siidorifera, 
occur  within  the  integument  of  all  parts  of  the  body,  with  the  exception  of 
that  covering  the  red  margins  of  the  Hps,  the  inner  surface  of  the  prepuce 
and  the  glans  penis.  They  are  especially  numerous  in  the  palms  and  soles, 
in  the  former  locality  numbering  more  than  iioo  to  the  square  centimeter, 
and  fewest  on  the  back  and  buttocks,  where  their  number  is  reduced  to 
about  60  to  the  square  centimeter;  their  usual  quota  for  the  same  area  is 
between  two  and  three  hundred. 

Modified  simple  tubular  in  type,  each  gland  consists  of  two  chief  divi- 
sions, the  body  or  gland-coil^  the  tortuously  wound  tube  in  which  secretion 
takes  place,  and  the  excretory  duct,  which  opens  on  the  surface  of  the  skin, 
exceptionally  into  a  hair-follicle,  by  a  minute  orifice,  the  sweat-pore,  often 
distinguishable  with  the  unaided  eye. 

The  body  of  the  gland,  irregularly  spherical  or  flattened,  consists  of 
the  windings  of  a  single  or  rarely  branched  tube.  It  commonly  occupies  the 
deeper  part  of  the  corium,  but  sometimes,  as  in  the  palm  and  scrotum,  lies 
within  the  subdermal  connective  tissue.  The  coiled  portion  of  the  gland  is 
not  entirely  formed  by  the  secretory  segment,  since,  as  shown  by  the  recon- 
structions of  Huber,  about  one  fourth  is  contributed  by  the  convolutions  of 
the  first  part  of  the  duct. 

The  secreting  portion  of  the  gland-coil,  called  the  ampulla  on  account 
of  its  greater  diameter,  possesses  a  wall  of  remarkable  structure.  The  thin 
external  sheath,  composed  of  a  layer  of  dense  fibrous  tissue  and  elastic  fibres, 
supports  a  well  defined  membrana  propria.  Immediately  within  the  latter 
lies  a  thin  but  compact  layer  of  involuntary  muscle,  whose  longitudinally  dis- 
posed spindle-shaped  elements  in  cross-section  appear  as  irregularly  nucleated 
cells  that  encircle  the  secreting  epithelium  and  displace  it  from  its  customary 
position  against  the  basement  membrane.  This  muscular  tissue  enjoys  the 
distinction,  which  it  shares  with  the  dilator  of  the  pupil,  of  being  developed 
from  the  ectoderm.  The  secreting  cells  constitute  a  single  row  of  low  col- 
umnar epithelial  elements,  that  lie  internal  to  the  muscle  and  surround  the 
relatively  large  lumen.  Their  finely  granular  cytoplasm  contains  a  spherical 
nucleus,  situated  near  the  base  of  the  cell,  and  in  certain  of  the  larger  glands, 
as  the  axillary,  includes  fat  droplets  and  pigment  granules.  These  are  liber- 
ated with  the  secretion  of  the  gland  and,  when  present  in  unusual  quantity, 
account  for  the  discoloration  produced  by  the  perspiration  of  certain  individ- 


THE   CUTANEOUS   GLANDS.  335 

uals.  In  the  case  of  the  ceruminous  glands,  the  amount  of  oil  and  pigment  is 
constantly  great  and  confers  the  distinguishing  characteristics  of  the  ear-wax. 
On  leaving  the  gland-coil,  in  close  proximity  to  the  blind  end  of  the 
gland,  the  duct  ascends  through  the  corium  with  a  fairly  straight  or 
slightly  wavy  course  as  far  as  the  epidermis.  On  entering  the  latter  its 
further  path  is  marked  by  conspicuous  corkscrew-like  windings,  which  ter- 
minate on  the  surface  by  a  trumpet-shaped  orifice,  the  sweat-bore.      In  its 


Spiral  part 

of  duct 


Stratum 
■  corneum 


.S.  lucidum 
-S.  granulosutn 

^  >s^i;':.  v^'^    :  ■  *'.         '-C''''^/  '■'    ''  ■?';^;'    ';''!  ■  '•    .' S.germiuativum 


v--cj^^.-^^vm 


}iy'-s^^m'l^> 


-^—■7-  Fat-cells 


l/.>W">-    ,  .        r-'^t'f&'^-^i-Dl'.   ;    '-n--   ^'''-""^     Coiled  part  of 

V',.^^.  "  -  ''_ ,    _?-*S*^^i,' _■■    '  '       '    ~ij— — — sweat-gland 


Fig.  381.  — Section  of  skin  from  palm,  showing  layers  of  epidermis  and  parts  of  sweat-glands  extending 
from  surface  into  tela  subcutaiiea.     ^^  65. 

course  through  the  corium  the  duct  never  traverses  a  papilla  or  ridge,  but 
always  enters  the  cuticle  between  the  elevations.  On  the  palms  and  soles, 
where  the  pores  occupy  the  summit  of  the  cutaneous  ridges,  the  ducts  enter 
the  cuticle  between  the  double  rows  of  papillae. 

The  sudden  and  conspicuous  reduction  in  the  size  of  the  tube,  which 
marks  the  termination  of  the  secreting  segment  and  the  beginning  of  the 
duct,  is  accompanied  by  changes  in  the  structure  of  its  wall.  In  addition  to 
a  reduction  of  its  diameter  to  one  half  or  less  of  that  of  the  ampulla,  the  duct 
loses  the  layer  of  muscle  and  becomes  flattened,  with  corresponding  changes 
in  the  form  of  its  lumen.  The  single  row  of  secreting  elements  is  replaced 
by  an  irregular  double  or  triple   layer  of    cuboidal  cells,   which  exhibit   a 


336  NORMAL    HISTOLOGY. 

homogeneous  zone,  sometimes  described  as  a  cuticle,  next  the  lumen. 
On  entering  the  epidermis,  the  duct  not  only  loses  its  fibrous  sheath  and 
membrana  propria,  but  the  epithelial  constituents  of  its  wall  are  soon 
lost  among  the  cells  of  the  stratum  germinativum,  so  that  its  lumen  is 
continued  to  the  surface  as  a  spiral  cleft  bounded  only  by  the  cornified  cells 
of  the  cuticle. 

Apart  from  mere  variations  in  size,  certain  glands — the  circumanal,  the 
ciliary^  and  the  ceriiminoiis — depart  sufficiently  from  the  typical  form  of  the 
coiled  glands  to  entitle  them  to  brief  notice.  The  circumanal  glands, 
lodged  chiefly  within  a  zone  from  12-15  mm.  wide  and  about  the  same  dis- 


}i  %i  \-J  Ml\ 


Basement 
/'Inembrane 


A 

^1     Muscle-cell 
:      Secreting-cells 

^^Parts  of  duct 


Parts  of 

Iciprrptino 


coiled 

secreting 
I  segment 


uscle-cells 


Fig.  382. — Section  of  coiled  portion  of  sweat-gland.     X  325. 

tance  from  the  anus,  are  not  all  the  same,  but  include,  according  to  Huber, 
four  varieties.  In  addition  to  (i)  the  usual  sweat-glands  and  (2)  some 
(Gay's)  of  exceptional  size,  (3)  others  have  relatively  straight  ducts  that 
end  in  expanded  saccules,  from  which  secondary  alveoli  arise;  finally  (4) 
branched  glands  of  the  tubo-alveolar  type  are  present.  The  ciliary  glands 
{ Molls' s)  of  the  eyelid  are  not  typical  coiled  structures,  but  belong  to  the 
branched  tubo-alveolar  groups.  The  ceruminous  glands,  distinguished 
by  the  large  amount  of  oil  and  pigment  mingled  with  their  secretion,  are 
likewise  referable  to  the  branched  tubo-alveolar  type. 

The  blood-vessels  of  the  sweat-glands  include  arterial  twigs  given  off 
from  the  cutaneous  rete,  a  capillary  network  outside  the  membrana  propria, 
best  developed  within  the  coiled  portion  of  the  tube,  and  the  veins  that  join 
the  deeper  plexus  within  the  corium. 

The  nerves  are  especially  numerous  and  consist  of  nonmedullated 
sympathetic  fibres  that  traverse  the  fibrous  sheath  and  form  a  close  epilem- 


THE    EYEBALL. 


337 


mar  plexus  on  the  outer  surface  of  the  membrana  propria.  From  this  net- 
work fibrils  penetrate  the  basement  membrane  and  end  in  close  relation  with 
the  gland-cells  and  muscle-elements. 


THE   EYE. 

The  organ  of  sight  proper  includes  only  the  eyeball  or  globe  of  the  eye; 
with  it,  however,  are  closely  associated  other  structures,  as  the  eyelids,  the 
lachrymal  apparatus,  the  orbital  fascia  and  fat,  and  the  ocular  muscles,  which 
serve  for  its  protection,  support  and  change  of  axis.  The  structure  of  the 
eyeball,  therefore,  will  be  clescribed  first;  afterwards,  that  of  some  of  the 
accessory  organs. 

THE   EYEBALL. 

The  human  eyeball  is  an  approximate  sphere  with  an  antero-posterior 
diameter  (24.2  mm.)  of  slightly  less  than  one  inch.  It  is,  however,  some- 
what flattened  from  above  downwards  and  from  side  to  side.      Its  shape. 


Crystalline  lens 
Suspensory  ligament  of  lens 
Canal  of  Schlemm 

Ciliary  body^ 
Conjunctiva 


Cornea 

Anterior  chamber 
Iris 
Posterior  chamber 

Sclero-corneal  junction 


Internal  muscle -/; ;    7/ 


Vena  vorticosa    +* 


External  muscle 


Vitreous 


Sclera 


Ciliary  nerve 


Posterior  ciliary  vessels. 

Hyaloid  canal      , 
Optic  nerve  - 

Central  retinal  vessels -^^ 


Fovea  centralis 
//(  '    ^Optic  papilla 

u 


Fig.  383. — Diagrammatic  horizontal  section  of  right  eye.     X  2^. 

therefore,  is  spheroidal,  with  the  vertical  diameter  (23.2  mm.)  the  shortest. 
The  eyeball  consists  of  three  concentric  tunics  or  coats:  (i)  the  exieriial  ox 
fibrous  tunic,  composed  of  the  sclera  and  the  cornea;  (2)  the  middle  or 
vasciUar  tunic,  which  is  pigmented,  partly  muscular  and  composed,  from 
behind  forwards,  of  the  choroid,  the  ciliary  body,  and  the  iris ;  and  (3)  the 
inner  or  nervous  tunic,  usually  called  the  retina,  which  is  an  expansion  from 
the  brain  and  contains  the  nerve-cells,  the  nerve-fibres  and  the  special  neuro- 
epithelium  for  the  reception  of  the  visual  stimuli.  Within  these  tunics  are 
enclosed  the  refracting  media — the  aqueous  humor,  the  crystalline  lens,  and 
the  vitreous  body. 


338 


NORMAL    HISTOLOGY. 
The  Fibrous  Tunic. 


The  Sclera. — The  sclera,  or  sclerotic  coat,  is  a  firm,  dense  fibrous 
tunic  which  forms  the  posterior  four-fifths  of  the  outer  coat  of  the  eye,  being 
closely  connected  with  the  sheaths  of  the  optic  nerve  posteriorly,  and  joining 
in  front  with  the  cornea.  In  the  neighborhood  of  the  optic  nerve  it  measures 
I  mm.  in  thickness,  gradually  becoming  thinner  towards  the  equator,  until, 
just  posterior  to  the  attachment  of  the  tendons  of  the  ocular  muscles,  it 
measures  only  .4  mm.  After  receiving  the  expansions  of  these  tendons  it 
increases  and  reaches  a  thickness  of .  6  mm.  The  optic  nerve  passes  through 
this  tunic  at  a  position  i  mm.  below  and  from  3-4  mm',  to  the  inner  side  of 


Fibre  layer 
Ganglion-cells 

ar  cells 
Visual  cells 


iSl^«*?^^?Sa 


Pigment  layer 

Stroma 
Large  vein 

Lamina  fusca 


Fibrous  tissue 
of  sclera 


'^^^^       Episcleral 

endothelium 
^^~~~~~-  Space  of  Tenon 
between  sclera 
and  capsule 
of  Tenon 


Fig.  384. — Section  through  posterior  wall  of  eyeball,  showing  relative  thickness  of  the  fibrous  vascular 

and  nervous  coats.     X  40. 

the  posterior  pole  of  the  eye;  the  canal  is  partially  bridged  over  by  inter- 
lacing fibrous  bundles,  the  lamina  cribrosa,  which  are  intimately  associated 
with  the  supporting  tissue  of  the  nerve. 

The  sclera  is  composed  of  interlacing  bundles  of  white  fibrous  tissue, 
which  in  the  outer  and  inner  layers  have  chiefly  a  meridional  direction,  while 
the  central  bundles  are  alternately  circular  and  meridional.  With  the  fibrous 
bundles  is  associated  a  rich  network  of  fine  elastic  fibres.  The  clefts  between 
the  lamelke  contain  irregularly  stellate  connective  tissue  cells,  the  scleral  cor- 
puscles. On  the  inner  surface  of  the  sclera  many  of  these  cells  are  pig- 
mented and  give  it  a  brownish  color.  This  layer,  the  lamina  ftisca,  with  the 
underlying  choroid  encloses  a  narrow  cleft,  the  S2iprachoroidal  lymph-space, 
both  walls  of  which,  together  with  the  fine  connective  tissue  trabeculae  which 
cross  it,  are  lined  with  endothelial  cells.  The  outer  surface  of  the  sclera, 
from  the  optic  nerve  entrance  to  the  attachment  of  the  ocular  muscles,  is 


THE    FIBROUS   TUNIC. 


339 


similarly  covered  with  endothelial  plates,  and  forms  part  of  the  lining  of 
Tenon's  lymph-space.  Anterior  to  the  muscle-insertions  it  is  covered  with 
a  loose-meshed  connective  tissue,  the  episcleral  tissue,  richly  supplied  with 
blood-vessels,  nerves  and  lymph-vessels,  and  continuous  with  the  subcon- 
junctival tissue  of  the  conjunctiva  sclercs. 

The  blood-vessels  of  the  sclera  arise  from  the  arteries  which  perforate  it 
to  supply  the  vascular  coat  of  the  eye.  They  form  a  wide-meshed  network 
on  the  surface  of  the  sclera,  which  sends  anastomosing  vessels  to  a  deeper 
lying  set  in  the  scleral  substance.  In  the  neighborhood  of  the  optic  nerve 
entrance,  the  branches  of  the  short  posterior  ciliary  arteries  form  an  arterial 
circle,  the  circulus  Zinni,  which  sends  branches  to  the  optic  nerve  and  cho- 
roid, and  is,  therefore,  of  great  importance  in  establishing  an  anastomosis 
between  the  choroidal  circulation  and  the  arteria  centralis  retince  which  sup- 
plies the  retina.  The  veins  of  the  sclera  empty  into  the  anterior  and  posterior 
ciliary  veins,  and    into  the 


venae  vorticosae.  At  the 
junction  of  the  cornea  and 
sclera  is  an  important  cir- 
cular venous  channel,  the 
canal  of  Schlemm,  which 
will  be  described  later. 

The  Cornea. —  The 
cornea  forms  the  anterior 
one-fifth  of  the  fibrous 
tunic  of  the  eyeball,  and, 
although  composed,  like 
the   sclera,    of    bundles    of 


Epithelium 


connective  tissue,  is  trans- 
parent and  allows  rays  of 
light  to  enter  the  eyeball. 

The  cornea  is  com- 
posed of  five  distinct  layers, 
which  from  without  in  are: 

( 1 )  the  anterior  epithelium, 

(2)  the  ayiterior  limiting 
membrane,  (3)  the  substan- 
tia propria,  (4)  the  poste- 
rior limiti^ig  membrane, 
and  (5)  the  endothelium. 

The  anterior  epithe- 
lium of  the  cornea  is  con- 
tinuous with  that  covering 
the  surface  of  the  adjacent  scleral  conjunctiva.  It  is  of  the  stratified 
squamous  variety,  usually  five  cells  deep  in  man,  and  measures  45  /x  in  thick- 
ness at  the  centre,  and  80  ,a  at  the  periphery.  The  deepest  cells  are  columnar 
in  form,  with  broad  bases  resting  upon  the  anterior  limiting  membrane,  to 
which  they  are  firmly  attached  by  means  of  minute  projections  that  roughen 
the  anterior  surface  of  the  latter.  The  outer  parts  of  the  basal  cells  contain 
the  nucleus  and  fit  into  corresponding  depressions  in  the  cells  of  the  super- 
imposed layers.  The  middle  layers  are  composed  of  irregular  polyhedral 
cells,  possessed  of  fine  protoplasmic  intercellular  processes.  The  superficial 
layers  consist  of  flattened  cells  which  lie  parallel  to  the  free  surface  and 
contain  nuclei. 


Fig.  385. — Section  of  human  cornea.     X  85. 


340  NORMAL    HISTOLOGY. 

The  anterior  limiting  membrane,  or  Bowman^ s  membrane^  is 
situated  immediately  below  the  epithelium  and  appears  as  a  homogeneous 
band,  about  20  /x  in  thickness  at  the  centre  and  thinner  at  the  periphery, 
where  it  terminates  without  extending  into  the  conjunctiva  of  the  sclera. 
The  membrane  may  be  resolved  into  fine  fibrillae  by  suitable  reagents,  is 
connected  firmly  with  the  cornea  proper,  and  is  to  be  considered  a  special 
condensation  of  the  latter.      It  contains  no  elastic  tissue. 

The  substantia  propria  constitutes  the  main  portion  of  the  cornea 
and  is  made  up  of  interlacing  bundles  of  fibrous  connective  tissue,  directly 
continuous  with  those  of  the  adjacent  sclera.  The  bundles  are  composed  of 
fine  fibrillae,  have  a  flattened  form,  and  are  so  disposed  as  to  produce  regular 
lamellae,  about  sixty  in  number,  running  parallel  with  the  surface.  The 
alternating  lamellae  have  a  direction  approximately  at  right  angles  to  each 
other  and  are  frequently  joined  together  by  strands,  Xh&Jibrce  araiatcs,  which 

are    especially   numerous 
^_,,-'  in   the  anterior   lamellae. 

'^  \^   /^  The  fibrillae  and  bundles 

►;'/  ^^         are  held  together  by  an 

''^'  '     '"         -  interfibrillar    cement-sub- 

"  ,        stance,  in  which  are  em- 

^,  ■^'^  \^^  .  i'       bedded   the   cellular  ele- 

"  '^  <■  ^    J^f     ^  1^"'*''         ments,  the  corneal  corpus- 

''K. '''  '"'■  ^    '  c/es.     These  are  flattened 

'-    "    ,J  '         '         connective     tissue    cells, 

^      "^        >-^  /  '^'ith  faintly  granular  cyto- 

/  '^'^ .     '     '  s  plasm,   whose  nuclei   are 

\  ,  ,<Y         f  '^*^s>.       irregular    and    show   nu- 

^         /     ^~~    ,^    .      ■■  "H^l*      cleoli.       The     cells     are 
./%  "  ,\J  '^       !        '         provided  with  branching 


J  ;;         _ ,         ^    ,  ^        .     -  processes   which    anasto- 

\^'     f '  *    /  '      ^;idl^  mose  with  those  of  other 

,     yAJ^' '"  ••   \  ^^'"^      cells,   both   on   the  same 

I    ^fv^^^  ^  J  ^j^(j   adjacent  levels,  and 

so  constitute  a  continuous 

Fig.  386. — Thin  sheet  of  corneal  tissue  stained  to  show  the  corneal  ,  i  i:    „      t    „i 

corpuscles  ;  surface  view.    X  350.  network  ^  OI     protoplasm, 

upon  which  the  nutrition 
of  the  cornea  largely  depends.  They  occupy  a  system  of  intercommunicat- 
ing lymph-clefts,  the  corneal  spaces,  which  during  life  they  fill  completely. 
Occasional  leucocytes  or  wandering  cells  are  found  between  the  fibrous 
bundles. 

The  posterior  limiting  membrane,  also  known  as  DescemeV s  mem- 
bra7ie,  the  7nembrane  of  Demours,  or  the  posterior  elastic  membrane,  is  a 
homogeneous  band,  which  varies  in  thickness  from  6  ij.  at  the  centre  to  12  /x 
at  the  periphery.  It  is  less  firmly  united  to  the  substantia  propria  than  is 
the  anterior  limiting  membrane,  and  is  less  easily  affected  by  acids,  alkalies, 
boiling  water  and  other  reagents.  It  resembles  elastic  tissue  and  is  very  firm 
and  resistent  to  injury  or  perforation  from  inflammation.  At  the  periphery 
the  membrane  splits  up  into  bundles  of  fine  fibres,  which  are  gradually 
strengthened  into  a  series  of  firm  connective  tissue  trabeculae.  Some  of 
these  form  the  point  of  attachment  of  the  ciliary  muscle;  others  run  into  the 
iris,  and  still  others  constitute  the  inner  wall  of  a  circularly  disposed  venous 
channel,  the  sinus  circidaris  iridis,  or  canal  of  Schlemm.  These  fibres  are 
known  as  the  liganientiun  pectinatiim  iridis  and  mark  the  lateral  limit  of  the 


1 


THE   VASCULAR   TUNIC. 


341 


Fig.  387. — Substantia  propria  stained  with  silver 
to  show  the  spaces  containing  the  corneal  cells. 

:■:  350- 


anterior  chamber.  They  are  incompletely  covered  with  endothelial  cells  and 
enclose  the  spaces  of  Fontana.  These  spaces,  better  developed  in  lower 
animals  than  in  man,  directly  communicate  with  the  aqueous  chamber,  and 
thus  form  an  important  point  for  filtra- 
tion of  fluid  from  the  interior  of  the 
eye,  by  way  of  the  canal  of  Schlemm, 
into  the  anterior  ciliary  veins. 

The  endothelium  covers  the 
free  inner  surface  of  the  posterior  limit- 
ing membrane.  It  consists  of  a  single 
layer  of  flat  polygonal  cells,  whose 
nuclei  often  extend  above  the  level  of 
the  cell-body.  The  cells  are  con- 
nected by  delicate  protoplasmic  proc- 
esses and  are  continuous  with  the 
cells  lining  the  spaces  of  Fontana  and 
the  anterior  surface  of  the  iris.  With 
Descemet'  s  membrane  they  constitute 
a  barrier  to  the  filtration  of  fluid  from 
the  anterior  chamber  into  the  cornea. 
The  blood-vessels  of  the  normal 
cornea  are  limited  to  a  peripheral 
zone,  from  1-2  mm.  in  width,  in  which  the  terminal  twigs  of  the  episcleral 
arteries  end  in  loops.  The  remainder  of  the  cornea  is  free  from  blood-vessels. 
The  nerves  of  the  cornea  are  exceedingly  numerous.     They  are  branches  of  the 

long  and  short  ciliary  nerves,  from 
40  to  45  in  number,  and  form  an 
annular  plexus  that  surrounds  the 
margin  of  the  cornea.  Entering 
the  latter,  they  are  accompanied 
for  a  short  distance  by  perineural 
lymph-sheaths  and,  losing  these 
and  their  medullary  substance,  they 
form  a  number  of  plexuses  within 
the  corneal  stroma  at  various 
depths.  A  few  of  the  fibres  pass 
backwards  and  supply  the  pos- 
terior layers.  Fully  two  thirds, 
however,  after  forming  a  fiinda- 
viental  plexus,  push  forwards  and 
send  perforating  branches  through 
the  anterior  limiting  membrane  and 
unite  into  a  subepithelial  plexus,  the  minute  radial  fibres  passing  towards  the 
centre  of  the  cornea.  From  this  plexus  fibrils  ascend  between  the  epithelial 
cells  and  end  either  as  varicose  fibrils,  or  in  connection  with  special  end-bulbs 
(the  intraepithelial  plexus).  After  forming  complex  secondary  plexuses, 
branches  from  the  fundamental  plexus  end  within  the  substantia  propria  as 
naked  fibrillae  between  the  lamellae. 


Canal 
/ 


'>»Sj. 


.,r 


V 


Spaces  of 
Fontana 


Trabeculae  of 
pectinate  ligament 


Bundles  of  ciliary  muscle 

Fig.  3S8. — Section  through  margin  of  anterior  cham- 
ber, showing  spaces  of  Fontana,  between  the  relaxed 
trabeculse  of  the  pectinate  ligament,  and  the  canal  of 
Schlemm.     X  65. 


The  Vascular  Tunic. 

The  middle  or  vascular  coat,  sometimes  called  tlie  uveal  tract,  consists 
of  a  connective  tissue  sheath  supporting  blood-vessels,  which  lies  internal  to 
the  outer  fibrous  tunic.      It  extends  from  the   entrance  of  the  optic  nerve 


342 


NORMAL   HISTOLOGY. 


to  the  pupil  and  includes  three  portions,  which  from  behind  forward  are:  the 

choroid,  the  ciliary  body,  and  the  iris.     The  choroid  and  ciliary  body  are  in 

contact  with  the  sclera,  but  the  iris  bends  sharply  inwards  and  floats  in  the 

aqueous  humor,  incompletely 

dividing    the    space    anterior  Membrana  vitrea^_^  ,«.--^- 

to    the    crystalline    lens    into 

the  posterior  and  the  anterior 

chamber. 

The  Choroid. — The  cho- 
roid contributes  the  posterior 
two-thirds  of  the  vascular  coat. 
It  lies  between  the  sclera  and 
the  retina  and  extends  from 
the  optic  nerve  entrance  to 
the  anterior  limit  of  the  visual 
part  of  the  retina  at  the  ora 

serrata,  its  main  function  being  to  supply  nutrition  to  the  nervous  tunic. 
It  is  a  delicate  coat,  with  a  thickness  of  .  2  mm.  near  the  nerve  and  about 
half  as  much  at  the  ora  serrata.     The  outer  surface  is  roughened  by  the 


Clioriocapillaris  ~ 


Large  vein  - 


Choroidal  stroma - 
Lamina  suprachorioidea- 
Supraclioroidal  space- 
Lamina  fusca  of  sclera- 


Fig.  389.- 


Section  of  choroid,  showina:  capillary  layer  and 
large  vessels.     X  200, 


Large  vein 


Artery 


Fig.  390. — Surface  view  of  injected  human  choroid,  showing  venous  radicles  converging  to  form  large 

vein.     X  iS. 

trabeculae  of  connective  tissue  which  cross  the  suprachoroidal  lymph-space 
and  connect  the  choroid  with  the  overlying  sclera.  Its  inner  S2irface 
is  smooth  and  covered  by  the  pigmented  cells  of  the  retina,  which  are  so 
closely  attached  that  they  frequently  adhere  to  the  choroid  when  the  mem- 
branes are  separated. 


THE   VASCULAR   TUNIC 


343 


,_^X*^ 


The  choroid  consists  of  four  layers,  which  from  without  inwards,  are: 
(i)  th^  lain?  na  siiprachorioidea,  (2)  the  choroid  proper,  which  contains  the 
larger  vessels,  (3)  the  choriocapillaris,  or  layer  of  iine  capillaries,  and  (4) 
the  membrana  vitrca. 

The  lamina  suprachorioidea  is  the  outer  boundary  of  the  choroid  and 
connects  it  with  the  sclera.  It  is  composed  of  interlacing  bundles  of  fibrous 
tissue,  which  are  strengthened  by  rich  networks  of  elastic  fibres.  The  cellular 
elements  consist  of  («)  flattened  endothelial  plates,  which  line  the  lymph- 
clefts  and  cover  the  connective  tissue  trabeculae  connecting  the  choroid  and 
the  sclera  ;  and  {b')  large  irregularly  branched  connective  tissue  cells,  the 
chromatophores,  which  are  conspicuous  on  account  of  their  deep  pigmentation. 
The  lamelhe  of  the  suprachoroid  continue,  without  definite  boundary,  into 
the  subjacent  choroidal  stroma. 

The  choroid  proper,  as  the  choroidal  stroma  is  called,  has  the  same 
general  structure  as  the  suprachoroidal  layer,  but  the  connective  tissue 
elements  are  denser  and  support  a  large  number  of  blood-vessels,  between 
which  are  placed  the  stellate  chromatophores.  The  largest  vessels  occupy 
the  outer  part  of  the  coat  and  are  chiefly  venous.  They  are  surrounded 
with  perivascular  lymph-sheaths,  and  converge  in  peculiar  whorls  to  form 
four  or  five  large  trunks,  the  vencz  vorticose,  which  pierce  the  sclera  in  the 
equatorial  region  and  drain  not  only  the  choroid,  but  partly  also  the  ciliary 
body  and  iris.  The  arteries,  derived  from  the  short  ciliary  vessels,  lie  internal 
to  the  veins.  Their  walls  contain  longi- 
tudinally disposed  muscle-fibres  in  addi- 
tion to  the  customary  circular  ones. 

The  choriocapillaris,  or  membrayie 
of  Riiysch,  is  composed  of  capillaries  which 
form  an  extremely  close  meshwork  em- 
bedded within  a  homogeneous  nonpig- 
mented  matrix.  Between  the  choriocap- 
illaris and  the  layer  of  larger  vessels  is  a 
narrow  boundary  zone  of  closely  woven 
fibro-elastic  strands,  which  is  nearly  free 
from  pigment.  In  some  animals  this  layer 
possesses  a  peculiar  metallic  reflex  and  is 
known  as  the  tapeium  fibrostcm;  in  carniv- 
ora  its  iridescent  appearance  is  due  to  the 
presence  of  cells  containing  minute  crystals 
{tapetian  celhilosum). 

The  membrana  vitrea,  or  viem- 
brane  of  Bruch,  the  innermost  layer  of  the 
choroid,  measures  only  2  ij.  in  thickness. 
It  separates  the  choriocapillaris  from  the  retina  and  consists  of  two  strata, 
an  inner  homogeneous  one,  probably  a  product  of  the  retinal  pigment  cells, 
and  an  outer  highly  elastic  portion.  The  nerves  of  the  choroid  form  a 
plexus  within  the  lamina  suprachorioidea,  which  contains  groups  of  ganglion- 
cells  and  sends  numerous  nonmeduUated  fibres  chiefly  to  the  muscular  coats 
of  the  arteries.      The  choroid  contains  no  sensory  nerve-fibres. 

The  Ciliary  Body. — The  ciliary  body,  the  middle  portion  of  the  vas- 
cular tunic,  extends  from  the  ora  serrata  to  the  sclero-corneal  junction. 
Sections  through  the  eyeball  in  a  meridional  direction  (Fig.  392)  show  its 
triangular  form.  The  outer  side  is  in  apposition  to  the  sclera,  the  inner  is 
covered  by  the  pigmented  extension  of  the  retina,  and  the  short  anterior 


Fig.  391. — Portion  of  injected  choroid,  show- 
ing surface  view  of  choriocapillaris  layer. 
X  130- 


344 


NORMAL   HISTOLOGY. 


side,  at  right  angles  to  the  outer,  extends  inwards  from  the  pectinate  hgament 
towards  the  lens.  The  ciliary  body  presents  three  subdivisions:  the  ciliary 
ring,  the  ciliary  processes,  and  the  ciliary  muscle. 

The  ciliary  ring,  or  orbicuhis  ciliaris,  consists  of  a  smooth  band  of 
tissue,  4  mm.  in  width,  in  advance  of  the  ora  serrata.  It  differs  in  structure 
from  the  choroid  in  the  absence  of  the  choriocapillaris,  its  vessels  running  in 
a  longitudinal  direction  and   returning    the  blood  from  the  iris  and  ciliary 


Cornea 


Canal  of  Schlemm 


Pectinate  ligament 


Pars  iridici. 
retir  a; 


muscle  (radial  fibres) 
clera 

Meridional  fibres 


Ciliary  processes  Circular  fibres  Choroid  Pars  ciliaris  retinae 

Fig.  392. — Meridional  section  of  ciliary  region,  showing  ciliary  body  with  its  muscle  and  processes.    X  40. 

body  to  the  venae  vorticosae.  On  its  inner  surface,  delicate  meridionally 
placed  folds  make  their  appearance,  by  the  union  of  which  the  ciliary 
processes  are  formed. 

The  ciliary  processes  constitute  the  remainder  of  the  inner,  portion 
of  the  ciliary  body.  They  form  an  annular  series  of  folds,  about  seventy  in 
number,  which  surround  the  lens  and  act  as  points  of  attachment  to  its  sus- 
pensory ligament.  Commencing  by  the  union  of  several  plications  of  the 
orbiculus  ciliaris,  they  rapidly  increase  in  height  and  breadth,  until  they  reach 
an  elevation  of  from  .8-1  mm.,  and  then  fall  suddenly  to  the  iris-level. 
They  consist  of  a  rich  network  of  vessels  embedded  in  a  pigmented  con- 
nective tissue  stroma,  like  that  of  the  choroid.  The  inner  surface  is  covered 
with  a  homogeneous  membrane,  continuous  with  the  membrana  vitrea  of  the 
choroid,  on  the  inner  surface  of  which  is  placed  the  double  layer  of  cells 
representing  the  ciliary  portion  of  the  retina  {pars  ciliaris  retincB).  Each 
ciliary  process  is  composed  of  a  number  of  irregularly  projecting  folds  which 
increase  in  height  as  the  iris  is  approached. 

The  ciliary  muscle  occupies  the  outer  portion  of  the  ciliary  body, 
lying  between  the  sclera  and  the  ciliary  processes.  It  forms  an  annular  pris- 
matic band  of  involuntary  muscle,  which  in  meridional  sections  has  a  trian- 
gular form.  Its  main  fibres  arise  from  the  sclera  and  pectinate  ligament,  at 
the  sclero-corneal  junction  internal  to  the  canal  of  Schlemm,  and  run  in  a 
meridional  direction  backwards  along  the  sclera  to  be  inserted  into  the  cho- 
roidal stroma.  The  inner  angle  of  the  triangle,  at  the  base  of  the  iris,  is 
occupied  by  a  band  of  circularly  disposed  fibres,  the  circular  ciliary  muscle 
of  Millie r.  Between  the  circular  and  meridional  portions,  the  fibres  assume 
a  radial  direction  and  are  separated  by  considerable  connective  tissue,  which 
in  the  deeply  pigmented  races  may  contain  many  branched  pigmented  cells, 
but  in  the  white  races  is  free  from  pigment.  Acting  from  its  origin,  the  cil- 
iary muscle  draws  forward  the  ciliary  processes  and  relaxes  the  lens-capsule. 


THE   VASCULAR   TUNIC. 


345 


Stroma 

Pigmented 

cells 


Clear  cells 

Blood-vessels 
in  processes 


The  blood-vessels  of  the   ciHary  body,  from   the  anterior  and  the  long 
ciHary  arteries,  form  a  ring  around  the  root  of  the  iris,  the  circidiis  arteriosus 
iridis  major,  from  which  \essels  are  sent  inwards  to  supply  the  iris,  ciliary 
muscle  and  ciliary  processes.      The  veins 
from  the  ciliary  muscle  empty  chiefly  into 
the  anterior  ciliary  veins;  those  from  the 
ciliary  processes  and  a  few'  from  the  ciliary 
muscle  pass  backwards  and  become  tribu- 
tary to  the  veyiee  vorticostz.     The  nerves 
of  the  ciliary  body  form  an  annular  plexus 
within    the    ciliary    muscle    and    include 
sensory  and  sympathetic  fibres,  the  latter 
being  distributed  to  the  walls  of  the  blood- 
vessels and  to  the  involuntary  muscle. 

The  Iris. — The  iris  forms  the  ante- 
rior segment  of  the  vascular  tunic  and  is 
visible  through  the  cornea.      Slightly  to 

the  inner  side  of  its  centre  is  placed  an  approximately  circular  opening,  the 
pupil.  The  periphery  of  the  iris  is  attached  to  the  ciliary  body  behind  and 
receives  fibres  from  the  pectinate  ligament  in  front.  The  color  of  the  iris 
varies  in  different  individuals  and  gives  the  "color  of  the  eye."  It  is 
dependent  partly  upon  the  amount  of  pigment  within  the  iris-stroma,  and 
partly  upon  the  density  of  the  pigmentation  of  the  cells  on  its  posterior  surface. 
In  light  blue  eyes,  the  stroma. contains  very  little  pigment  and  the  posterior 
pigment  layer,  seen  through  it,  gi\'es  a  bluish  tint;  whereas  in  brown  eyes 
the  stroma  contains  so  much  pigment  that  the  posterior  pigment  layer  is 
totally  obscured  and  the  iris  appears  brown. 

The  stroma  of  the  iris  encloses  numerous  thick-walled  blood-vessels, 
radiating  from  the  ciKary  border  towards  the  pupil.  They  are  supported  by 
a  delicate  connective  tissue  framework,  which  contains  irregularly  shaped, 


Fig.  393. — Section  of  ciliarv'  processes,  sliow- 
ngr   layers  of   ciliary  part  of  nervous  tunic. 


Pupillary  margin 


Anterior  endothelium        Stroma  of  iris 


Sphincter  muscle  Pigmented  retinal  layer 

Fig.  394. — Section  of  pupillary  end  of  iris.    X  210. 

branching  pigmented  cells,  many  nerves  and  lymph-spaces.  The  anterior 
surface  is  covered  with  a  single  layer  of  polygonal  endothelial  cells,  con- 
tinuous with  those  lining  the  cornea.  Beneath  these  cells  is  a  condensation 
of  the  connective  tissue  stroma,  the  anterior  boimdary  layer,  in  which 
the  cells  are  closely  placed.  Minute  clefts  in  the  tissue  form  a  direct 
communication  between  the  anterior  chamber  and  the  interfascicular  lymph- 
clefts.  In  very  dark  irides  pigment  is  found  not  only  within  the  branched 
cells,  but  heaped  in  irregular  masses  within  the  stroma.  The  muscular 
tissue  of  the  iris  includes  two  distinct  masses,  the  sphincter  and  dilatator 
of  the  pupil. 


346 


NORMAL   HISTOLOGY. 


Pupillary   border, 
of  iris 


Arterial  circle 


Ciliary  processes 


The  sphincter  imisde  is  a  band  of  involuntary  tissue  about  .7  mm.  in 
width,  surrounding  the  pupil  and  situated  in  the  vascular  stroma,  back  of  the 
blood-vessels. 

The  dilatator  muscle  is  a  sheet  of  smooth  muscle-fibres  in  the  position 
formerly  described  as  the  posterior  limiting  lamella,  or  membrane  of  Bruch. 

Investigations  have  settled 
definitely  the  question  of  its 
existence  and  shown  that  its 
fibres  arise  from  the  outer 
cells  of  the  retinal  pigment 
layer,  on  the  posterior  sur- 
face of  the  iris.  They  do 
not  reach  quite  to  the  pupil- 
lary border. 

The  posterior  surface 
of  the  iris  is  covered  by  the 
pigmented  layer,  which 
morphologically  represents 
the  anterior  segment  of  the 
atrophic  nervous  tunic  {pars 
iridica  retincB^ .  This  is  con- 
tinuous with  the  pigmentary 
layer  covering  the  ciliary 
processes,  but  the  cells,  dis- 
posed as  a  double  layer,  are 
so  deeply  pigmented  as  to 
be  indistinguishable  without 
bleaching  the  tissue.  Since 
the  dilatator  muscle  is  de- 
veloped from  the  fusiform 
cells  of  the  outer  layer  it  rep- 
resents an  epithelial  (ecto- 
dermic)  muscle.  The  inner 
cells  are  larger  polygonal  elements,  which  gradually  lose  their  pigment  as 
they  approach  the  ciliary  processes.  Over  the  latter  they  contain  no  pig- 
ment, whereas  the  outer  cells  remain  pigmented. 

The  blood-vessels  of  the  iris  pass  radially  inwards  from  the  circtdus  arteri- 
osus iridis  major  at  the  periphery.  Near  the  pupillary  border,  they  form  a 
second  ring,  the  circulus  arteriosus  iridis  minor,  branches  from  which 
supply  the  sphincter  muscle  and  the  pupillary  zone.  The  venous  radicles 
unite  to  form  trunks  which  accompany  those  from  the  ciliary  processes  to 
empty  into  the  ve7i(z  vorticosce.  The  lymphatics  are  represented  by  the 
interfascicular  clefts  which  communicate  with  the  anterior  chamber,  with  the 
spaces  within  the  ciliary  body,  and  with  the  spaces  of  Fontana.  The  7ierves 
of  the  iris,  branches  of  the  ciliary  nerves,  follow  the  course  of  the  blood- 
vessels and,  branching,  form  a  plexus  of  nonmeduUated  fibres,  which  supply 
chiefly  the  involuntary  muscle,  including  that  of  the  vessels. 


Choroidal  -^ 

veins 

Fig.  395. — Injected  ciliary  processes  and  iris  ;  posterior  surface. 
X  20. 


The   Nervous   Tunic. 

The  Retina. — The  retina,  the  light-perceiving  portion  of  the  eye, 
represents  a  modified  portion  of  the  brain  itself,  with  which  it  develops  in 
close  connection.      It  is  a  delicate  membrane  and  extends  from  the  optic 


THE   NERVOUS   TUNIC. 


347 


nerve  entrance  to  the  pupillary  border.  The  functioning  portion,  or  pars 
optica  retiyics,  reaches  as  far  forwards  as  an  irregular  wavy  line,  the  07-a 
sn-rata  ;  anterior  to  this,  the  retina  is  represented  by  an  atrophic  portion, 
consisting  of  a  double  layer  of  cells  covering  the  ciliary  body  and  the  iris, 
respectively  the  pars  ciliaj'is  and  pars  iridica  rctince.  The  pars  optica 
retinae  is  closely  applied  to  the  inner  surface  of  the  choroid  and  is  in  contact 
with  the  hyaloid  membrane  investing  the  vitreous  body.      It  gradually  di- 


Coriiea 


Greater  arterial  ring 
Iris 

Lesser  arterial  ring 

Ciliary  process 


Canal  of  Schlemni 

'Corneal  loop 


Perforating  branch 

Conjunctival  vessels 


Anterior  ciliary 
vessels 


Sclera 
Episcleral  vessels 


Communication  between  Retinal  vessels 

choroidal  and  optic  vessels 
Central  retinal  vessels 


Vena  vorticosa 

Supplying  choroid 
Short  post.  cil.  art. 

Long  post.  cil.  art. 
Communicating  twig 
Inner  sheath  vessels 
Outer  sheath  vessels 

-Communicating  vessels 
Fig.  3g6. — Diagram  illustrating  circulation  of  eyeball.     {Leber.) 

minishes  in  thickness  from  .4  mm.  at  the  posterior  pole  to  .1  mm.  near  the 
ora  serrata.  At  the  posterior  pole  of  the  eyeball,  3  mm.  to  the  outer  side 
of  the  optic  nerve  entrance,  the  retina  exhibits  an  oval  area,  the  yellow  spot 
ox  macula  I  idea ;  the  centre  of  the  latter  is  marked  by  a  small  depression, 
the  fovea  centralis,  which  corresponds  to  the  region  of  sharpest  vision. 

The  retina  is  composed  of  nervous  elements  which  are  supported  by  a 
specialized  sustentacular  tissue  or  neuroglia.  Morphologically  it  must  be 
considered  as  composed  of  two  lamellae,  which  correspond  to  the  outer  and 
inner  walls  of  the  optic  vesicle,  the  hollow  outgrowth  of  the  brain-sac  from 


348 


NORMAL   HISTOLOGY. 


which  it  is  developed.  The  fundamental  divisions  of  the  retina  are:  (i)  the 
external  lamella,  the  pigmented  layer  on  the  outer  surface;  and  (2)  the 
internal  lamella,  which  includes  the  remaining  layers  of  the  retina.  The 
inner  lamella  may  be  subdivided  further  into  the  neuroepithelial  .and  the 
cerebral Xdiy^xs.  Sections  of  the  retina  (Fig.  397)  show  under  the  microscope 
from  without  inwards  the  following  layers: 


Outer  Lamella  of  Optic 
Vesicle 


II.  Inner  Lamella  of  Optic 
Vesicle 


{'■ 


Pigmented  layer 


>  Pigmented  layer 


'  2.  Layer  of  rods  and  cones 

3.  Layer  of  bodies  of  visual  cells,  or 

outer  nuclear  layer 

4.  Outer  plexiform  layer 

<j  5.   Layer  of    bipolar  cells,    or   inner 
nuclear  layer 

6.  Inner  plexiform  layer 

7.  Layer  of  ganglion-cells 
L  8.  Layer  of  nerve-fibres 


Neuro- 
epithelial 
layer 


^  Cerebral 
layer 


Fibre  of  Miiller 


Internal  limiting: 
membrane 


Ganglion  cell 


Fibres  of  Miiller 

Inner  plexiform 
layer 


Bipolar  nerve 
cells 


Blood-vessel 


To  these  nervous  layers  must  be  added  two  delicate  membranes,  (i)  the 

-inembrana  limitans  intertia,  which  bounds  the  inner  surface  of  the  retina, 

and  (2)  the  membrana  lim- 
itans exter7ia,  which  lies  be- 
tween the  outer  nuclear 
layer  and  the  layer  of  rods 
and  cones.  These  mem- 
branes represent  the  termi- 
nal portions  of  the  support- 
ing neurogliar  fibres,  or 
fibres  of  Miiller. 

The  pigmented  layer, 
formed  of  deeply  pigmented 
cells,  constitutes  the  most 
external  layer  of  the  retina 
and  represents  the  outer 
wall  of  the  foetal  optic  vesi- 
cle. It  is  composed  of 
hexagonal  cells,  from  12— 
1 8  /x  in  diameter,  the  proto- 
plasm of  which  is  loaded 
with  fine,  needle-shaped 
crystals  of  pigment  (yfus- 
cin).  The  outer  portion  of 
the  cells  is  almost  free  from 
pigment  and  contains  the 
nucleus.  From  the  inner 
border  fine  protoplasmic 
processes  extend  inwards 
between  the  rods  and  cones 
of  the  neuroepithelial  layer, 
and  under  the  influence  of 
light  the  pigment  particles 

wander  into  these  processes  and  thus  surround  the  percipient  elements. 

The  layer  of  rods  and  cones,  although  usually  described  as  a  distinct 

stratum,  is  only  the  highly  specialized  outer  zone  of  the  layer  of  visual  cells 


Outer  plexiform 
layer 


Layer  of  visual 
cells 


Nucleus  of  cone 

cell 


Pigment  la>er 


Fig. 


397. — Sect  on  of  human  ret  na,  near  posterior  pole  of 

eyeball.     X  230. 


THE   NERVOUS   TUNIC. 


349 


A6 


Fig.  398. — Pigmented  cells 
from  outer  layer  of  retina; 
surface  view.    X  350. 


and,  therefore,  constitutes  the  outer  portion  of  the  neuroepitheHal  division  of 
the  retina.  It  is  composed,  as  its  name  indicates,  of  two  elements,  the  rods 
and  the  cones,  which  are  the  outer  ends  of  the  rod-  and  cone-visual  cells. 
They  are  closely  set,  with  their  long  axes  perpen- 
dicular to  the  surface  of  the  retina.  The  rods  far 
outnumber  the  cones,  except  in  the  fovea  centralis, 
in  which  location  cones  alone  are  found.  In  the 
macula  each  cone  is  surrounded  by  a  layer  of  rods; 
elsewhere  the  cones  are  separated  by  intervals  occu- 
pied by  three  or  four  cones. 

The  rods  of  the  human  retina  (Fig.  399,  B)  have 
an  elongated  cylindrical  form  and  measure  approxi- 
mately 60  //  in  length  and  2  /x  in  diameter.  Each  rod 
is  composed  of  an  outer  and  an  inner  segment,  of 
about  equal  length.  The  outer  segment  possesses 
a  uniform  diameter,  is  doubly  refracting,  and  readily 
breaks  up  into  minute  disks.  It  is  invested  with  a  delicate  covering  of 
neurokeratin,  contains  inveloid  and  is  the  situation  of  the  visual  pnrple 
that  tinges  the  living  retina.  The  inner  rod-segment  is  somewhat  thicker 
and  has  an  ellipsoidal  form.  It  is  singly  refracting,  homogeneous  in  structure 
and  from  its  inner  extremity  sends  the  delicate  rod-Jibre  through  the  external 
limiting  membrane  into  the  outer  nuclear  layer, 
where  the  nucleus  of  the  rod-visual  cell  is  found. 

The  cone-visual  cell  is  composed  of  the  same 
general  divisions  as  the  rod-cell,  including  the 
specialized  outer  part,  the  cone,  and  the  body  within 
tlae  external  nuclear  layer.  The  cones  are  shorter 
than  the  rods  and  have  a  length  of  35  ij-.  Each  one 
(Fig.  399,  A)  is  composed  of  an  outer  narrow  cone- 
shaped  segment,  and  an  inner  broader  one,  which 
is  distinctly  ellipsoidal,  with  a  diameter  of  7  p..  The 
inner  segment  is  double  the  length  of  the  outer  and 
continuous,  as  the  cone-fibre,  with  its  nucleus  in  the 
outer  nuclear  layer. 

The  outer  nuclear  layer,  the  inner  portion 
of  the  neuroepithelial  layer,  is  about  60  /^  thick  and 
composed  of  the  bodies  of  the  rod-  and  cone-visual 
cells,  which  show  chiefly  as  the  nuclei,  the  rod-  and 
cone-granules.  The  rod-granules  occupy  an  ellip- 
tical enlargement  of  the  attenuated  rod-hbres, 
exhibit  a  transverse  striation  and  are  placed  at 
varying  levels  within  the  layer.  The  rod-fibres  are 
continued  as  a  thin  protoplasmic  process  into  the 
outer  reticular  layer,  where  they  form  small  end- 
knobs  which  are  associated  with  the  outer  terminals 
of  the  small  nerve-cells,  the  rod-bipolars.  The  cone- 
gra?niles  are  less  numerous  than  those  of  the  rods, 
display  no  transverse  markings,  and  are  found  only  in  the  outer  portion  of  the 
nuclear  layer.  The  cone-fibres,  the  attenuated  bodies  of  the  cone-visual  cells, 
are  broader  than  the  corresponding  parts  of  the  rods  and  continued  through 
the  outer  nuclear  layer  as  far  as  the  outer  portion  of  the  external  plexiform 
layer.  Here  they  end  with  broad  bases,  from  which  delicate  processes  extend 
inwards  to  interlace  with  the  terminal  arborizations  of  the  cone-bipolars. 


/ c  Im 


Fig.  399. — Visual  cells  from 
human  retina;  A,  cone-cell,  B, 
rod-cell ;  a,  b,  outer  and  inner 
segments  ;  c,  attenuated  bodies 
(fibres),  with  nucleus  (rf)  and 
central  ends  (e)\  ^ /;«,  external 
limiting  membrane.     {Greeff.) 


350  NORMAL   HISTOLOGY. 

The  outer  plexiform  layer  is  a  narrow  granular  looking  stratum, 
between  the  outer  and  the  inner  nuclear  layer,  and  constitutes  the  first  of  the 
cerebral  layers  of  the  retina.  It  is  composed  of  the  dendritic  arborizations 
of  the  bipolar  nerve- cells  of  the  succeeding  layer,  which  lie  in  close  relation 
with  the  foot-plates  of  the  cone-cells  and  with  the  end-knobs  of  the  rod-fibres. 

The  inner  nuclear  layer,  the  most  complicated  of  the  retinal  strata, 
measures  35  p.  in  thickness,  near  the  optic  disk.  It  contains  nervous  elements 
of  three  main  types — the  horizontal  cells,  the  bipolar  cells,  and  the  amacrine 
cells. 

The  horizontal  cells  have  flattened  cell-bodies  and  send  out  dendrites, 
which  terminate  in  close  association  with  the  bases  of  the  rod-  and  cone-visual 
cells.  Each  horizontal  cell  possesses  also  an  axone,  which  ends  in  a  richly 
branched  arborization  about  the  visual  cells.  The  function  of  the  horizontal 
cells  is  not  well  understood,  but  they  probably  serve  as  association  fibres 
between  the  visual  cells. 

The  bipolar  cells,  the  ganglion- cells  of  this  layer,  are  of  two  chief 
varieties,  the  rod-bipolars  and  the  cone-bipolars.  They  are  oval  cells,  each 
sending  an  axone  inwards,  which  ends  in  relation  with  the  large  nerve-cells  of 
the  ganglion-cell  layer,  and  a  dendrite  outwards,  which  is  associated  with  the 
visual  cells. 

The  amacrine  cells  are  placed  in  the  inner  portion  of  the  nuclear  layer. 
They  are  nerve-cells,  although  no  distinct  axone  can  be  demonstrated. 
They  possess,  however,  richly  branched  dendritic  processes,  which  ramify 
in  the  inner  plexiform  layer. 

The  inner  plexiform  layer,  40  /z  thick,  appears  granular,  similar  to 
the  corresponding  outer  zone,  and  is  composed  of  the  interlacing  axones  of 
the  bipolar,  amacrine  and  horizontal  cells  from  the  inner  nuclear  layer  and 
the  dendrites  of  the  large  ganglion-cells  in  the  subjacent  retinal  layer.  Inter- 
mingled with  these  are  the  fibres  of  Miiller,  which  show  as  conspicuous 
vertical  striae,  with  lateral  of?shoots. 

The  layer  of  ganglion-cells  consists,  throughout  the  greater  part  of 
the  retina,  of  a  single  row  of  large  multipolar  neurones,  each  with  a  cell- 
body  containing  a  vesicular  nucleus  and  nucleolus  and  exhibiting  typical 
Nissl  bodies  and  a  fibrillar  structure.  Their  axones  pass  inwards  and  become 
the  nerve-fibres  of  the  fibre  layer.  Converging  towards  the  optic  entrance, 
they  become  consolidated  into  the  optic  nerve  and  continue  to  the  brain.  The 
dendrites  of  the  ganglion-cells,  one  to  three  in  number,  run  outwards  into  the 
inner  plexiform  layer  and  end  as  richly  branched  arborizations  in  connection 
with  the  centrally  directed  processes  from  the  bipolar  cells. 

The  nerve-fibre  layer  is  composed  almost  entirely,  but  not  exclusively, 
of  the  axones  of  the  ganglion-cells  of  the  preceding  layer.  The  individual 
fibres  are  collected  into  bundles  of  varying  size,  which  take  a  horizontal 
course  and  converge  towards  the  optic  disk.  Within  the  retina  they  are 
devoid  of  medullary  sheaths,  but  acquire  them  after  passing  through  the 
lamina  cribrosa  of  the  sclera.  A  few  of  the  fibres  are  centrifugal,  arising 
from  ganglion-cells  within  the  brain,  and  terminate  apparently  in  relation 
with  the  amacrines  of  the  inner  nuclear  layer. 

The  sustentacular  tissue,  the  neuroglia  of  the  retina,  exists  in  two 
forms — as  \\-\(t  fibres  of  Miiller  and  the  spider  cells. 

The  fibres  of  Miiller  are  modified  neuroglia-fibres,  which  pass  ver- 
tically from  the  inner  surface  of  the  retina  through  the  succeeding  layers  as 
far  as  the  bases  of  the  rods  and  cones  (Fig.  400).  The  inner  extremities  of 
the   fibres  are  conical  expansions,  that  by  apposition  form  an  incomplete 


THE   NERVOUS   TUNIC. 


351 


sheet,  the  membrana  limitans  interna.  As  the  fibres  traverse  the  retinal 
layers,  they  give  off  delicate  lateral  offshoots,  which  break  up  into  a  fine 
supporting  reticulum.  Within  the  inner  nuclear  layer  each  fibre  presents  a 
broad  expansion  containing  the  oval  nucleus  of  the  sustentacular  cell.      After 


Sustentacular  cell 
Fibre  of  Muller 


Neuroepithelial  layer 


Pigmented  layer 


Inner  nuclear  layer 
{bipolar  iierve-c«lls) 


^T^  Outer  plexiform  layer 


Outer  nuclear  layer 
(bodies  of  visual  cellsi 


Rods  and  cones 


^■■^^BHMMPigmented  layer 
Choroid 

Fig.  400. — Diagram  illustrating  structure  of  retina  and  relations  of  three  fundamental  layers.     {Greeff.) 


traversing  the  outer  nuclear  layer  their  broadened  peripheral  ends  come  into 
contact  and  form  a  continuous  sheet,  the  vionbrana  limitans  externa.  From 
the  latter  delicate  offshoots  continue  outwards  and  embrace  the  bases  of  the 
individual  rods  and  cones.  In  addition  to  the  robust  fibres  of  Miiller,  neu- 
roglia cells,  in  the  form  of  spider  cells,  are  found  in  the  nerve-fibre  and 
ganglion-cell  layers. 

The  Macula  Lutea. — The  structure  of  the  retina  undergoes  important 
modifications  in  two  areas,  at  the  macula  lutea  and  at  the  ora  serrata.      In  the 


7^v:?^,sa::r; 


/—f 


"  3  'f  i  -^ 

Fig.  401. — Section  of  human  retina  through  the  fovea  centralis,  i,  fovea;  2,  3,  cones  and  nuclei  of 
visual  cells;  4,  pigmented  layer;  5,  outer  plexiform  layer;  6,  bipolar  cells;  7,  ganglion-cells;  8,  inner 
plexiform  layer;  9,  internal  limiting  membrane.     ;•;  80. 

former  the  ganglion-cells  increase  rapidly  in  number  as  the  macula  is  reached, 
so  that  instead  of  forming  a  single  layer  they  are  distributed  in  from  eight  to 
ten  strata.  The  inner  nuclear  layer  is  also  increased  in  thickness.  Within 
the  fovea  centralis,  however,  in  order  to  reduce  to  a  minimum  the  layers 
traversed  by  the  light-rays,  the  cerebral  layers  are  almost  entirely  displaced. 


352 


NORMAL   HISTOLOGY 


only  the  absolutely  essential  retinal  strata — the  pigment  cells  and  the  visual 
cells  with  their  necessary  connections — being  retained  within  the  area  of 
sharpest  vision  (Fig.  401).  On  approaching  the  fovea,  the  ganglion-cells 
rapidly  decrease  in  number,  until,  at  the  centre  of  the  depression,  they  and 
the  nerve-fibre  layer  are  entirely  absent.  The  bipolar  cells  are  present  as  an 
irregular  layer  within  the  fused  remains  of  the  two  plexiform  layers.  The 
most  conspicuous  elements  are  the  visual  cells,  in  this  position  represented 
solely  by  the  cones,  that  have  about  twice  their  usual  length  and  thickness, 
the  increase  in  length  being  contributed  by  the  outer  segments.  The  cone- 
cell  nuclei  become  removed  from  the  external  limiting  membrane;  the  cone- 
fibres  are  therefore  lengthened,  pursue  a  radial  direction,  and  constitute  the 
so-C2^Q.A  fibre-layer  of  Henle.  Opposite  the  centre  of  the  fovea,  the  choroid 
is  thickened  by  an  increase  in  the  choriocapillaris.  The  yellow  color  of  the 
macula  is  due  to  a  diffuse  coloration  of  the  inner  retinal  layers. 

The  Ora  Serrata. — The  visual  part  of  the  retina  ends  anteriorly  in 
an  irregular  line,  the  ora  serrata.  The  retina  diminishes  in  thickness  in  con- 
sequence of  the  abrupt  disappearance  of  its  nervous  elements.  The  rods 
disappear  first;  then  the  cones  become  rudimentary,  and  finally  cease;  the 
ganglion-cells,  nerve-fibre  layer  and  inner  plexiform  layer  fuse,  and  the  two 
nuclear  layers  unite  and  lose  their  characteristics,  most  of  the  nuclei  present 
being  those  of  the  supporting  fibres  of  Miiller,  which  are  here  highly  devel- 
oped. These  elements  continue  beyond  the  ora  serrata  (Fig.  402)  as  the 
transparent  cylindrical  cells  composing  the  inner  layer  of  the  pars  ciliaris 


Vacuole 


Pigmented  cells 
"      Inner  cells 


Bipolar  cells 


Fig.  402.— Section  of  human  retina  through  ora  serrata,  showing  transition  of  pars  optica  into 

ciliaris.     X  li)"^. 


pars 


retincE,  the  densely  pigmented  cells  of  the  outer  layer  being  a  direct  contin- 
uation of  the  retinal  pigmented  cells.  These  two  strata  of  cells  are  pro- 
longed over  the  ciliary  body  and  the  iris  as  far  as  the  pupil,  over  the  iris 
constituting  the  pars  iridica  retin(Z.  As  the  columnar  cells  pass  forwards, 
they  gradually  decrease  in  height,  and  at  the  junction  of  the  ciliary  body 
and  the  iris  the  cells  of  both  layers  become  deeply  pigmented. 

The  blood-vessels  of  the  retina  are  derived  from  the  central  artery, 
which  enters  the  optic  nerve  behind  the  eyeball,  and,  with  its  accompanying 
vein,  runs  in  the  axis  of  the  nerve  until  it  emerges  slightly  to  the  nasal  side 
of  the  centre  of  the  optic  disk.  Here  the  artery  divides  into  two  short  supe- 
rior and  inferior  branches,  each  of  which  subdivides  into  nasal  and  temporal 
branches  which  give  off  end-arteries,  no  anastomosis  existing.  The  macular 
region  is  supplied  by  special  twigs,  the  centre  of  the  fovea,  however,  being 
free  from  blood-vessels.  The  larger  branches  course  within  the  nerve-fibre 
layer,  and  send  fine  twigs  to  form  an  inner  and  an  outer  plexus,  the  former 
on  the  outer  surface  of  the  inner  plexiform  layer,  and  the  latter  within  the 
inner  nuclear  layer.  Beyond  the  outer  plexiform  layer  the  vessels  do  not 
penetrate,  the  visual  cells  being  dependent  for  their  nourishment  upon  the 
choriocapillaris  of  the  choroid.     The  lymphatics  of  the  retina  are  repre- 


THE    OPTIC    NERVP:.  353 

seated  chiefly  by  the  perivascular  lymph-spaces  which  surround  the  blood- 
vessels. These  spaces  may  be  injected  from  the  subpial  lymph-space  of  the 
optic  nerve,  and  by  the  same  method  communications  may  be  demonstrated 
between  ( i )  this  space  and  the  interstices  between  the  nerve-bundles  which 
converge  towards  the  optic  papilla,  (2)  a  space  between  the  membrana 
limitans  interna  and  the  hyaloid  membrane  of  the  vitreous,  and  (3)  a  narrow 
cleft  between  the  pigmented  cells  and  the  layer  of  rods  and  cones. 

The  Optic  Nerve. — The  optic  nerve  is  surrounded  by  the  three 
sheaths — the  dural,  the  arachnoidal,  and  the  pial — which,  with  the  subdural 
and  the  subarachnoidal  lymph-spaces,  are  continued  over  the  nerve  as  pro- 
longations of  the  corresponding  brain-membranes.  On  reaching  the  eyeball, 
the  dural  sheath  bends  directly  outwards,  its  fibres  commingling  with  those 
of  the  outer  third  of  the  sclera  (Fig.  403);  the  arachnoid  ends  abruptly  on 

Physiological  excavation        1  imiii  i  ci  ibro';  i 

Fibre  layer—    "  —    ' 

Retina --^^^SSsss^Ek^^        ^N\\  O'^       /      ;<S^^=sSs^«s^f*^£ 
Choroid— ^•■J-.gA^>-''"t-  '*'^-      ^  ,^       \SJ^',''      /Lar'^v"* 

Sclera ^ 


Dural  sheath \  /   ;  '  Subarachnoidal 

space 


Archnoidal_ ^ ^ 

sheath 


Pial  sheath - 


Subdural  space 


Central  retinal  vessels  within  optic  nerve 
Fic;.  403.— Section  of  eyeball  through  entrance  of  optic  nerve.       '  20. 

the  inner  wall  of  the  intervaginal  space;  whilst  the  pia  arches  outwards  to 
form  part  of  the  inner  third  of  the  sclera,  sending  longitudinal  fibres  as  far 
as  the  choroid.  As  the  nerve-fibres  enter  the  eyeball,  for  convenience 
assuming  that  they  are  passing  from  the  brain  towards  the  retina,  they  trav- 
erse a  fenestrated  membrane,  the  lamina  cribrosa,  which  is  formed  by 
interlacing  bundles  from  the  inner  third  of  the  sclera  and  from  the  pial 
sheath.  As  the  nerve-fibres  penetrate  the  lamina  cribrosa  they  lose  their 
medullary  sheaths  and,  in  consequence,  the  optic  nerve  is  reduced  one 
third  in  diameter.  The  nerve  projects  slightly  into  the  eyeball  on  account 
of  the  thickness  of  the  layer  of  arching  nerve-fibres  and  forms,  therefore,  a 
circular  elevation,  known  as  the  optic  papilla,  about  1.5  mm.  in  diam- 
eter, the  centre  of  which  is  modelled  by  a  funnel-shaped  depression,  the 
so-called  physiological  excavation.  The  axis  of  the  nerve  is  occupied  by  the 
central  artery  of.  the  retina,  which  gives  oflt  minute  branches  for  the  nutrition 
of  the  nerve,  that  anastomose  with  the  pial  vessels,  and,  through  the  circulus 
arteriosus  Zinni,  with  branches  of  the  posterior  ciliary  arteries.  In  trans- 
verse sections  (Fig.  404),  the  optic  nerve  appears  as  a  mosaic  of  irregular 
polygonal  areas  composed  of  bundles  of  meduUated  nerve-fibres  surrounded 
by  connective  tissue  envelopes.  Although  provided  with  medullary  sheaths, 
the  optic    fibres    are    devoid   of    a  neurilemma,    in    this    respect  agreeing 

23 


354 


NORMAL    HISTOLOGY. 


with  the  nerve-fibres  composing   the  central  nervous  system.     The  entire 
nerve  corresponds  to  a  huge  funiculus,  the  perineurium  being  represented 


Blood-vessel- 


-J^^^- 


fe-^    .    . 


'"    <  .< 


Bundles  of.J4 
nerve-fibres^    ^ 


Interfascicular 
connective  tissue  ~? 


^  "^     -    '-^^v'L. 


Fig.  404. — Transverse  section  of  part  of  optic  nerve,  showing  bundles  of  nerve-fibres.     X  125. 

by  the  pial  sheath,  and  the  endoneurium  by  the  interfascicular  septa  of  con- 
nective tissue  prolonged  from  the  pia  between  the  bundles  of  fibres.  Nu- 
merous connective  tissue  cells  occur  along:  the  strands  of  fibrous  tissue. 


The  Crystalline  Lens. 

The  lens,  the  most  important  part  of  the  refractive  apparatus  of  the  eye, 
is  a  biconvex  body  suspended  from  the  ciliary  body  by  the  suspensory  liga- 
ment or  zonule  of  Zinn.  Its  anterior  surface  supports  the  pupillary  margin 
of  the  iris,  its  posterior  surface  resting  in  a  depression,  the  patellai-  fossa^  on 
the  anterior  surface  of  the  vitreous  body.      It  is  completely  transparent  and 

enclosed    in    a    transparent    elastic 
^  ^  membrane,  the  lens  capsule.    Togeth- 

er with  the  capsule,  the  lens  measures 
from  9-10  mm.  in  its  transverse 
diameter,  and  about  4  mm.  in  thick- 
ness from  pole  to  pole. 

The  capsule,  which  entirely 
surrounds  the  lens,  is  a  transparent, 
structureless,  highly  elastic  mem- 
brane, which,  while-  resistent  to 
chemical  reagents,  cuts  easily  and 
then  rolls  outwards.  It  is  thickest 
on  the  anterior  surface,  where  it 
measures  from  10-15//.,  ^'^d  thinnest 
at  the  posterior  pole  (5-7  //).  In  the 
adult  the  lens  is  devoid  of  blood- 
vessels, but  during  a  part  of  foetal 
life  it  is  surrounded  by  a  vascular  network,  the  tunica  vasculosa  lentis,  which 
is  supplied  chiefly  by  the  hyaloid  artery.  This  temporary  vessel  is  the  ter- 
minal branch  of  the  central  artery  of  the  retina  and  passes  from  the  optic 
disk  forwards  through  the  hyaloid  canal  or  canal  of  Cloquet  in  the  vitreous  to 
the  surface  of  the  lens  (Fig.  383).      The  vascular  lens-tunic  and  the  hyaloid 


Fig.  405. — Fibres  of  crystalline  lens ;  A,  B,  frag- 
ments of  isolated  fibres ;  C,  fibres  in  cross-section. 
X  275. 


THE   VITREOUS    BODY. 


355 


Fic.  406. — Portion  of  lens  and 
its  capsule;  A,  section  throu<;Ii 
equator;  a.  capsule  ;  d,  epithelial 
cells,  which  at  z  transform  into 
lens-fibres  (/)  with  nuclei  («);  J}. 
fragment  of  capsule  (h)  with  epi- 
thelium {e),  surface  view.    X  130. 


artery  are  temporary  structures  and  usually  disappear  before  birth.  Excep- 
tionally they  may  persist,  the  tunic  being-  represented  by  the  pupillary  mem- 
brane and  the  artery  by  a  fibrous  strand  within  the 
vitreous,  stretching  from  the  optic  disk  towards 
the  lens. 

The  anterior  portion  of  the  capsule  is  lined  by 
a  single  layer  of  f^at  polygonal  cells,  the  epithelium 
of  the  lens  capsule,  which  represents  morphologi- 
cally the  anterior  wall  of  the  original  lens-vesicle. 
On  approaching  the  equator  of  the  lens,  these  cells 
become  elongated  and  gradually  converted  into 
the  young  lens-fibres,  whose  nuclei  lie  in  the  super- 
ficial part  of  the  lens. 

The  lens-substance  is  composed  of  long 
flattened  fibres,  in  cross-sections  of  compressed 
hexagonal  outline,  5-1 1  //  broad  and  2—4  //  thick, 
held  together  by  an  interfibrillar  cement-substance. 
These  fibres  are  modified  epithelial  elements, 
which  develop  by  the  elongation  of  the  original 
ectodermic  cells  of  the  posterior  layer  of  the  primary 
lens-\'esicle.  The  subsequent  growth  of  the  lens 
depends  upon  a  similar  modification  of  the  anterior 
capsule-cells,  the  region  where  this  transformation  occurs  being  known  as 
the  transitional  zone.  The  individual  lens-fibres  vary  greatly  in  length, 
those  forming  the  outer  layers  being  longer  and  thicker  than  those  which 
constitute  the  centre  of  the  lens.  The  edges  of  the  fibres  are  finely 
serrated,  and,  as  the  points  of  the  serrations  of  adjacent  fibres  are  in  contact, 
fine  intercellular  channels  are  left  for  the  passage  of  nutritive  fiuid. 

The  Vitreous  Body. 

The  vitreous  body  fills  the  space  between  the  lens  and  the  retina,  being 
in  close  contact  with  the  retina  and  acting  as  a  support  to  it  as  far  forwards 

^  as    the   ora    serrata.      Here    it 

/ — ' '^35^^-;ii_/  ,  ,  separates  from  the  retina  and 
passes  to  the  posterior  surface 
of  the  lens,  presenting  a  shallow 
depression,  the  hyaloid  or  pa- 
tellar fossa,  on  its  anterior 
surface  for  the  reception  of  the 
lens.  The  fresh  vitreous  is 
semifluid,  perfectly  transparent 
and  consists  of  about  98.5  per 
cent,  of  water. 

The  vitreous  possesses 
a  framework  of  delicate  un- 
branched  fibrils,  which  pass  in 
all  directions  through  the  vitre- 
ous space  and  form  the  meshes 
in  which  the  fluid  constituents 
of  the  mass  are  held.  The 
surface  of  the  vitreous  is  en- 
closed by  a  delicate  boundary  layer,  called  the  hyaloid  membrane, 
formed  by  condensations  of  the  fibrils,  arranged  parallel  to  the  surface  and 


Fig.  407. — Portion   of    vitreous   body,  showing  feltwork  ot 
fibres  and  remains  of  cells       x   450.     {Retzius.) 


J56 


NORMAL    HISTOLOGY. 


closely  felted.  It  is,  however,  not  a  true  membrane,  but  only  a  condensa- 
tion of  the  vitreous  fibres.  The  vitreous  is  attached  firmly  to  the  retina  at 
the  nerve-entrance  and  at  the  ora  serrata,  between  these  points  the  hyaloid 
being  indistinct.  As  the  vitreous  leaves  the  retina,  the  boundary  layer  be- 
comes thicker,  in  some  cases  to  become  thin  again  or  absent  in  the  region  of 
the  patellar  fossa.  The. adult  vitreous  ordinarily  contains  no  cells,  but  some 
small  round  ones  are  occasionally  seen  near  the  surface,  beneath  or  on  the 
hyaloid  membrane.  They  are  amoeboid,  often  contain  vacuoles  and  are 
modified  leucocytes.  In  addition  a  few  branched  connective  tissue  cells  may 
be  present,  as  the  remains  of  the  mesodermic  elements  gaining  entrance 
along  with  the  blood-vessels  during  foetal  life.  The  central  part  of  the  vit- 
reous is  occupied  by  a  channel,  the  hyaloid  canal,  also  known  as  the  canal 
of  Stilling  or  the  canal  of  Cloquet,  which  is  about  i  mm.  wide  and  extends 
from  the  optic  entrance  towards  the  posterior  pole  of  the  lens.  During  foetal 
life  this  canal  lodges  the  arteria/iyaloidea,  the  continuation  of  the  central 
artery  of  the  retina. 

The  Suspensory  Apparatus  of  the  Lens. 

The  lens  is  held  in  position  by  a  series  of  delicate  bands,  which  pass 
from  the  vicinity  of  the  ora  serrata  over  the  ciliary  processes  to  be  attached 
to  the  periphery  of  the  lens.      These  fibres  collectively  constitute  the  sus- 


Cornea. 
Canal  of  Sclilemm        ^ 


Sclera 


Iris 


Ciliary 
muscle 


Ciliary  Suspensory 

processes    ligament  of  lens 


Fig.  408.— Meridional  section  of   ciliary  region,  showing  ciliary  processes  and  suspensory  ligament  of 

lens.    X  20. 

pensory  ligament,  or  zonule  of  Zinn,  a  structure  of  importance  not  only 
for  the  support  of  the  lens,  but  also  in  assisting  the  ciliary  muscle  in  effect- 
ing the  changes  in  the  curvature  of  the  lens  incident  to  accommodation.  The 
zonule  is  not,  as  formerly  believed,  a  continuous  membrane,  but  is  composed 
of  a  complicated  system  of  fibres.  The  latter,  varying  in  thickness  from 
5-22//.,  arise  chiefly  from  the  cuticular  membrane  covering  the  pars  ciliaris 
retinae  in  the  vicinity  of  the  ora  serrata.  Some  fibres  arise  also  from  the 
membrana  limitans  interna  of  the  retina,  whilst  others  end  within  the  vitre- 
ous body.  The  greater  number  of  the  fibres  pass  forwards  in  the  depressions 
between  and  along  the  sides  of  the  ciliary  processes;  they  then  proceed  in- 


THE   AQUEOUS    CHAMBER.  357 

wards  across  the  circunilental  space  to  be  attached  to  the  capsule  of  the  lens. 
Some  of  the  fibres  are  inserted  anterior  to  the  equator,  others  posterior  to 
the  equator,  and  some  directly  into  the  lens  margin.  Those  inserted  ante- 
riorly arise  behind  and  chiefly  from  the  valleys  between  the  ciliary  processes, 
whilst  those  inserted  back  of  the  equator  come  from  the  ciliary  processes  in 
front.  As  they  diverge  to  gain  their  insertion  in  the  lens-capsule,  the  cross- 
ing fibres  enclose  an  annular  space,  triangular  in  section,  whose  base  is  di- 
rected towards  the  lens  cquat<jr.  The  fibres  are  so  closely  interlaced  that  it 
is  possible  to  inject  air  between  them  and  so  produce  a  beaded  ring  surround- 
ing the  lens.  This  appearance  was  long  interpreted  as  demonstrating  the 
presence  of  a  delicate  channel,  the  canal  of  Petit,  encircling  the  lens.  The 
existence  of  a  definite  channel,  however,  is  no  longer  accepted,  the  space 
capable  of  inflation  being  part  of  the  larger  circunilental  space,  which  is 
filled  with  fluid  and  communicates,  by  means  of  fine  clefts,  with  the  posterior 
chamber. 

The  AcjuEuis  Humor  and  its  Ch.vmber. 

The  aqueous  humor  is  the  transparent  fluid  which  fills  the  space  between 
the  anterior  surface  of  the  vitreous  body  and  the  posterior  surface  of  the 
cornea.  In  chemical  composition  it  closely  resembles  water,  containing  only 
traces  of  albumin  and  extractives,  and  differing  from  Ij^iiph  in  its  low  per- 
centage of  albumin.  It  is  derived  chiefly  from  the  blood-vessels  of  the  cili- 
ary .processes  by  the  action  of  the  double  layer  of  cells  covering  the  pars 
ciliaris  retinae.  The  aqueous  humor,  constantly  produced,  is  carried  off 
through  the  spaces  of  Fontana  into  the  canal  of  Schlemm,  and  also  through 
the  lymph-spaces  in  the  iris.  With  the  exception  of  a  few  migratorv  leuco- 
cytes, the  aqueous  humor  is  devoid  of  morphological  elements. 

The  space  occupied  by  the  aqueous  humor  is  incompletely  subdivided 
by  the  iris  into  two  compartments,  the  anterior  and  posterior  chambers  of 
the  eye.  The  anterior  chamber  is  bounded  in  front  by  the  cornea  and 
behind  by  the  iris  and  lens,  and  has  a  depth  at  its  centre  of  from  7.5-8.5 
mm.  The  posterior  chamber  is  the  small  annular  space,  triangular  in 
cross-section,  which  has  for  its  anterior  boundary  the  iris,  is  limited  laterallv 
by  the  ciliary  processes,  and  medially  and  posteriorly  by  the  lens  and  the 
vitreous  body.  The  spaces  between  the  fibres  of  the  suspensory  ligament 
communicate  with  the  posterior  chamber,  are  filled  with  aqueous  humor, 
and  are,  therefore,  only  a  part  of  the  posterior  chamber. 

THE    EYELIDS   AND    CONJUNCTIVA. 

The  eyelids  or  palpebrce  are  two  movable  folds  of  integument — an 
upper  and  a  lower — strengthened  along  their  free  margins  by  a  lamina  of 
dense  fibrous  tissue,  the  tarsal  plate,  and  modified  on  their  deeper  aspect  so 
that  this  surface  resembles  a  mucous  membrane,  the  conjunctiva.  The  free 
border  of  the  lid  presents  a  well-defined  posterior  margin,  along  which  open 
the  minute  ducts  of  the  tarsal  glands,  whilst  the  anterior  margin  is  rounded 
and  passes  insensibly  into  the  adjoining  external  skin-surface  and  is  beset 
with  the  eyelashes.  The  latter,  the  cilia,  are  stifi  outwardly  curving  hairs, 
which  number  from  100-150  in  the  upper  lid  and  about  half  as  many  in  the 
lower.  That  part  of  the  narrow  conjunctival  sac  which  co\ers  the  posterior 
surface  of  the  lids  constitutes  the  palpebral  conjicnctiva  and  that  reflected 
onto  the  eyeball  is  the  biclbar  conjunctiva,  while  the  bottom  of  the  groove, 
where  these  two  portions  are  continuous,  is  known  as  "Cd^  fornix  conjunctivae. 
The  Inchrymal  lake  is  the  shallow  bay  into  which  the  conjuncti\'al  sac  is  pro- 


358  NORMAL    HISTOLOGY. 

longed  for  about  5  mm.  between  the  medial  ends  of  the  eyelids.  It  contains 
an  irregularly  oval  or  comet-shaped  elevation,  the  lachrymal  caritnde.  The 
latter  consists  of  an  islet  of  modified  skin  from  which  project  usually  about  a 
dozen  minute  and  scarcely  visible  hairs,  provided  with  large  sebaceous  and 
smaller  sweat-glands  and  embedded  in  a  cushion  of  fatty  tissue.  Just  to  the 
outer  side  of  the  caruncle,  lies  a  vertical  crescentic  fold,  the  plica  semilu- 
naris, which  frequently  contains  a  minute  plate  of  hyaline  cartilage  as  the 
vestige  of  the  stronger  bar  in  the  nictitating  membrane,  which  the  fold  repre- 
sents. Likewise  the  small  group  of  alveoli  sometimes  found  within  the  base 
of  the  fold  is  regarded  as  the  homologue  of  the  Harderian  gland.  Where  the 
boundaries  of  the  lachrymal  lake  pass  into  the  edges  of  the  eyelids  are  little 
elevations,  the  lachrymal  papillcz,  each  of  which  is  pierced  by  a  minute 
aperture,  the  pu7ictum  lacrimalis,  that  marks  the  beginning  of  the  canals  by 
which  the  tears  are  normally  carried  off  from  the  conjunctival  sac. 

The  Eyelids. — The  eyelid  comprises  five  layers  which,  from  without 
inwards,  are:  (i)  the  skin,  (2)  the  subcutaneous  tissue,  (3)  the  muscular 
layer,  (4)  the  tar so-fascial  layer,  and  (5)  the  conjimctiva. 

The  skin  covering  the  outer  surface  of  the  eyelids  is  characterized  by 
its  unusual  delicacy,  being  thin  and  beset  with  very  fine  downy  and  widely 
scattered  hairs,  provided  with  sebaceous  follicles;  small  sweat-glands  also  occur. 

The  subcutaneous  tissue  is  distinguished  by  the  entire  absence  of  fat, 
its  loose  texture  and  great  extensibility  and  elasticity.  In  consequence  of 
these  properties,  it  sometimes  becomes  the  seat  of  extensive  swelling  after 
edema  or  hemorrhage. 

The  muscular  layer,  for  the  most  part  the  annular  bundles  of  the 
orbicularis  palpebrarum,  is  so  blended  with  the  subcutaneous  tissue  as  to  be 
practically  emlDedded  within  the  latter.  In  vertical  sections  of  the  eyelid, 
(Fig.  409)  the  circularly  arranged  muscular  bundles  show  as  transversely 
cut  groups  of  muscle-fibres  enclosed  by  condensations  of  the  surrounding 
areolar  tissue.  A  distinct  annular  tract,  the  ciliary  bundle  or  muscle  of 
Riolan,  lies  close  to  the  free  border  of  the  lid,  chiefly  between  the  tarsal 
plate  and  the  hair-follicles,  in  part  often  also  between  the  conjunctiva  and 
the  tarsus.  In  the  upper  lid,  in  addition  to  the  circular  bundles  of  the  orbic- 
ularis palpebrarum,  the  terminal  strands  of  the  longitudinal  fibres  from  the 
levator  palpebrae  superioris  descend  along  the  deeper  surface  of  the  first- 
named  muscle.  Some  of  these  penetrate  between  the  circular  bundles  and 
end  in  the  deeper  layer  of  the  skin;  others  descend  more  vertically  to  find 
their  insertion  in  the  upper  border  of  the  tarsal  plate.  Under  the  name 
tarsal  muscles  or  muscles  of  Miiller,  are  described  the  uncertain  bundles  of 
involuntary  muscle  found  in  the  vicinity  of  the  convex  borders  of  the  tarsi. 
Those  within  the  upper  lid  arise  from  the  tendon  and  intermingle  with  the 
fibres  of  the  levator  palpebrarum  and  insert  either  into  the  upper  border  of 
the  tarsal  plate  or  into  the  adjacent  fibrous  tissue.  In  the  lower  lid,  they 
are  less  numerous  and  regular,  and  extend  from  the  fornix  conjunctivae  to 
the  adjacent  border  of  the  tarsus. 

The  tarso-fascial  layer  is  represented  next  the  margins  of  the  lids 
by  the  tarsal  plates  and  beyond  the  latter  by  a  dense  fascial  sheet. 

The  tarsal  plates  are  two  crescentic  lamellae  of  dense  fibrous  tissue, 
one  in  each  lid,  that  occupy  the  margins  of  the  eyelids,  to  the  maintenance 
of  whose  form  they  largely  contribute.  The  upper  tarsus  is  the  larger.  The 
plates  are  approximately  i  mm.  in  thickness  and  consist  of  densely  felted 
fibrous  tissue.  They  are  blended  in  front  and  below  with  the  subcutaneous 
tissue,   above  with  the  orbital  fascia  and  the  insertion  of  the  lid-muscles. 


THE    EYELIDS. 


359 


and  behind  with  the  conjuncti\a.  The  tarsal  plates  lodge  the  linear  series 
of  the  Meibomian  or  tarsal  glands.  These  structures,  between  thirty 
and  fortv  in  number  in  the  upper  lid  and  about  one  third  less  in  the  lower 
one,  consist  of  a  chief  tubular  duct,  placed  vertically  and  lined  by  stratified 
squamous  epithelium,  which  is  beset  with  numerous  simple  or  branched, 
irregular,  flask-shaped  alveoli.  The  latter  contain  cuboidal  epithelial  ele- 
ments that  resemble  in  appearance  and  condition  those  found  in  sebaceous 
follicles,  to  which  class,  in  fact,  the  tarsal  glands  belong.     They  secrete  an 


gkin 


Subcutaneous  tissue 

Orbicularis  palpebrarum 


■^,,       ^^' 


/      Tarsal  muscle 

/  Levator  palpebrse 


'^. 


'JV.'^^fA^. 


_>-^lood- vessel 


Henle's  gland 


w 


Meibomian  gland 


?T  Conjv 


|>^:;:;/ 


Duct  in  tarsal  plate 


Arterj-  of  tarsal  arch 


eibomian  duct 


Glands  of  Moll  Cilia  Ciliary  bundle 

Fig.  409. — Sagittal  section  of  upper  eyelid  of  child.     X  i5- 

oily  substance,  sebum  palpebrarum,  which  is  discharged  through  the  minute 
punctiform  orifices  of  the  ducts  seen  as  a  row  of  dark  points  just  external 
to  the  sharp  conjunctival  border  of  the  eyelid.  In  this  manner  the  latter  is 
kept  lubricated,  and  thus,  under  usual  conditions,  maintains  an  effective 
barrier  against  the  overflow  of  the  tears  from  the  conjunctival  sac.  Within 
the  free  edge  of  the  eyelids,  just  in  advance  of  the  tarsal  plates,  lie  the 
glands  of  Moll  and  the  glands  of  Zeiss.  The  former  are  coiled  tubules, 
resembling  modified  sweat-glands,  the  latter  sebaceous  glands,  the  ducts  of 
which  usually  open  close  to  or  into  the  mouths  of  the  follicles  of  the  eye-lashes. 


36o  NORMAL    HISTOLOGY. 

The  palpebral  conjunctiva  lines  the  ocular  surface  of  the  eyelids. 
Since  the  latter  are  developed  as  integumentary  folds,  at  first  the  conjunctiva 
resembles  the  skin,  but  after  the  temporary  closure  of  the  lids,  from  the 
middle  of  the  third  month  until  shortly  before  birth,  it  loses  its  original 
character,  and  later,  bathed  continuously  with  the  secretion  of  the  tear-gland, 
assumes  the  general  appearance  of  a  mucous  membrane.  Over  the  tarsi  the 
palpebral  conjunctiva  is  so  tightly  adherent  to  the  underlying  hbrous  plate, 
that  the  tunica  propria  is  reduced  to  an  insignificant  layer  and  the  Meibomian 
glands  shimmer  through  the  smooth  translucent  conjunctiva  and  appear  as 
parallel  stripes.  On  gaining  the  convex  border  of  the  tarsal  plates,  the 
conjunctiva  becomes  loose  and  movable  since  the  tunica  propria,  which  here 
connects  the  epithelium  with  the  underlying  fascial  tissue,  is  plentiful.  The 
small  tubular  glands  of  Henle  often  occupy  the  subepithelial  tissue  of  this 
part  of  the  conjunctiva.  In  the  fornix  and  its  vicinity  minute  lymph-nod ulcs 
occur,  either  discrete  or  in  small  groups.  In  the  same  locality  and  at  the 
convex  borders  of  the  tarsi,  small  nests  of  serous  alveoli,  known  as  accessory 
tear-glands,  ox  glands  of  Kraiise,  are  found.  They  are  much  more  numerous 
in  the  upper  than  in  the  lower  lid. 

The  bulbar  conjunctiva  passes  from  the  fornix  onto  the  anterior  part 
of  the  eyeball,  over  which  it  extends  as  far  as  the  corneal  margin,  at  which 
point  (Jimbus  cornecB^  the  tunica  propria  ends  and  the  epithelium  alone  con- 
tinues uninterruptedly  over  the  cornea.  During  its  passage  from  the  free 
edge  of  the  eyelid  to  the  cornea,  the  character  of  the  conjunctival  epithelium 
varies  in  different  parts  of  the  sac.  Thus,  at  the  border  of  the  lids  and  for 
a  few  millimeters  over  the  tarsi,  it  resembles  the  epidermis  in  being  stratified 
squamous.  Towards  the  convex  border  of  the  tarsal  plates  the  squamous  type 
gives  place  to  the  cylindrical;  in  the  retrotarsal  fossa,  throughout  the  fornix 
and  for  a  short  distance  over  the  eyeball,  the  epithelium  is  exclusively  columnar, 
varying  in  thickness  and  in  the  number  of  its  layers;  while  over  the  cornea 
and  adjacent  parts  of  the  sclera,  the  epithelium  is  again  stratified  squamous. 

The  blood-vessels  form  an  arch  in  each  lid  along  the  base  of  each 
tarsus,  between  the  latter  and  the  orbicularis  muscle,  from  which  perforating 
twigs  penetrate  the  tarsal  plates  for  the  supply  of  the  Meibomian  glands  and 
adjacent  conjunctiva.  The  lymphatics  are  arranged  in  two  sets,  a  pretarsal 
and  a  post-tarsal,  the  networks  of  which  are  connected  by  vessels  which 
pierce  the  tarsi.  The  former  receives  lymph  from  the  skin  and  muscles,  the 
latter  from  the  Meibomian  glands  and  the  conjunctiva.  The  nerves  sup- 
plying the  eyelid  include  sensory,  motor  and  sympathetic  fibres.  The  main 
branches  lie  between  the  tarsi  and  the  orbicularis  muscle,  sending  branches 
forwards  to  the  skin  and  backwards  through  the  tarsi  to  the  Meibomian  glands 
and  the  conjunctiva.  Those  to  the  conjunctiva  lose  their  medullary  coat  and 
terminate  either  in  free  arborizations,  beneath  or  among  the  epithelial  cells, 
or  in  the  end-bulbs.  The  latter  are  particularly  numerous  along  the  lid- 
margin,  but  occur  also  in  the  palpebral  and  bulbar  conjunctiva  and  at  the 
corneal  border.  The  sympathetic  fibres  supply  the  tarsal  and  other  lid- 
glands  and  send  filaments  to  the  walls  of  the  blood-vessels. 

THE   LACHRYMAL   APPARATUS. 

The  lachrymal  apparatus  consists  of  the  gland  secreting  the  tears, 
situated  in  the  anterior  and  outer  portion  of  the  orbital  cavity,  and  the 
system  of  canals  by  which  the  tears  are  conveyed  from  the  mesial  portion  of 
the  conjunctival  sac  to  the  inferior  nasal  meatus. 


THE    LACHRYMAL   APPARATUS.  361 

The  lachrymal  gland  resembles  in  shape  and  si/e  a  small  almond  and 
consists  of  two  fairly  distinct  parts,  the  superior  orbital  portion  and  the 
inferior  palpebral  or  accessory  portion.  The  former  occupies  the  fossa 
lacrimalis  in  the  frontal  bone  and  is  the  larger  portion,  measuring  20  mm.  in 


Alveoli 


Beginning  of  duct 


Fig.  410. — Section  of  laclirynial  gland,  showing  general  arrangement  of  alveoli.     X  ao. 

length  and  12  mm.  in  breadth.  The  lower  portion  of  \[\q. '^■a.v^A,  glanditla 
lacrimalis  i?iferior,  is  smaller  than  the  upper  and  separated  from  the  latter 
by  a  fascial  expansion. 

The  ducts  from  both  portions  of  the  gland  are  exceedingly  fine,  those 
from  the  upper  portion,  from  three  to  six  in  number,  passing  downwards 
through  the  inferior  portion.  Some  of  the  ducts  from  the  lower  gland  join 
those  coming  from  above,  while  others  run  independently.  They  are  lined 
with  a  double  layer  of  columnar  epithelial  cells.  In  all  about  a  dozen  ducts 
open  into  the  conjunctival  sac  along  a  line  just  in  front  of  the  fornix.  In 
structure  the  glands  correspond  to  the  tubo-alveolar  type  and  resemble  the 
serous  glands  in  their  general  character.  The  tubular  alveoli  contain  cells  of 
two  kinds — columnar  elements  in  which  the  stored  secretion  particles  occupy 
the  inner  half  of  the  cells,  and  low  elements  whose  cytoplasm  may  be  almost 
completely  filled  with  secretory  products.  The  ah'eoli  of  the  lower  portion 
are  separated  by  robust  septa  of  connecti\'e  tissue,  A\hich  contain  consider- 
able lymphoid  tissue. 

Accessory  lachrymal  glands  are  found  in  both  the  upper  and  lower 
fornices,  from  eight  to  thirty  being  present  in  the  upper  lid  and  from  two 
to  four  in  the  lower.  They  are  very  small  and  situated  chiefly  near  the  outer 
angle  of  the  palpebral  fissure. 

The  lachrymal  passages  begin  by  minute  openings,  the  lachrymal 
puncta,  which  are  usually  placed  at  the  summit  of  the  conical  lachrymal 
papillce.  The  puncta  lead  into  the  lachrymal  canaliciili,  which  at  first  are 
vertically  directed,  then  bend  abruptly,  take  a  nearly  horizontal  course, 
and  empty  into  the  lachrymal  sac.  Each  canaliculus  is  from  8-10  mm.  in 
length.  Its  lumen  measures  only  .  i  mm.  in  diameter  at  the  punctum, 
presents  a  diverticulum   of    i    mm.    at  the    bend,    and   continues  with    an 


362  NORMAL    HISTOLOGY. 

approximacely  uniform  calibre  of  .5  mm.  in  its  horizontal  portion.  The 
canalicukis  possesses  a  Hning  of  stratified  squamous  epithelium,  which  rests 
upon  a  delicate  tunica  propria  rich  in  elastic  fibres,  muscular  fibres  from  the 

orbicularis  palpebrarum  affording  additional  sup- 
port. The  muscle-bundles  run  parallel  to  the 
horizontal  portion  of  the  canaliculus,  but  are 
arranged  as  a  circular  sphincter  about  the  vertical 
portion. 

The  lachrymal  sac  may  be  regarded  as 
the  upper  dilated  portion  of  the  naso-lachrymal 
duct,  the  lower  part  of  which  passes  through  a 
bony  canal  and  opens  into  the  inferior  nasal 
meatus.  The  sac  is  about  15  mm.  long,  and 
6  mm.  in  diameter  when  distended.  The  wall 
of  the  sac  is  composed  of  a  fibro-elastic  tunica 
Fig.  4n.-Aiyeoii  of  lachrymal  gland    propria    of   lymphoid  character  and   is    loosely 

more  highly  magiuned.     X  235.  1       ■  1       1  •  •    1 

connected  with  the  periosteum  by  a  stratum  rich 
in  veins.  A  few  small  branched  tubular  glands  are  usually  present.  It  is 
lined  with  a  double  layer  of  columnar  epithelial  cells,  which  in  part  are 
provided  with  cilia. 

The  naso-lachrymal  duct,  the  lower  portion  of  the  tear-passage, 
varies  from  12-24  mm.  in  length,  and  is  from  3-4  mm.  in  diameter.  The 
mucous  membrane  lining  the  duct  is  clothed  with  columnar  epithelium 
and  may  contain  small  glands  in  the  lower  portion.  It  is  separated  from 
the  periosteum  by  areolar  tissue  and  a  venous  plexus. 

THE   EAR. 

The  auditory  organ  is  conventionally  described  as  the  external,  middle 
and  internal  ear — structures  lodged  entirely  or  in  part  within  the  tem- 
poral bone.  The  external  ear  includes  the  auricle  and  the  external  auditory 
canal;  the  middle  ear  the  tympanum,  the  Eustachian  tube  and  the  mastoid 
cells;  and  the  internal  ear  the  labyrinth,  with  the  peripheral  ramifications  of 
the  auditory  nerve.  Such  division,  moreover,  is  justified  by  the  develop- 
mental history  of  the  organ,  since  the  internal  ear  is  developed  essentially 
from  the  highly  differentiated  otic  vesicle  which  gives  rise  to  the  complicated 
membranous  labyrinth;  the  middle  ear  largely  from  the  first  pharyngeal 
pouch;  whilst  the  external  ear  represents  the  deepened  and  modified  boun- 
daries of  the  first  external  visceral  furrow. 


THE   EXTERNAL   EAR. 

The  external  ear,  the  outermost  subdivision  of  the  auditory  organ,  in- 
cludes (i)  the  auricle,  the  funnel-shaped  appendage  attached  to  the  side  of 
the  head  for  the  collection  of  the  sound-waves,  and  (2)  the  external  auditory 
canal,  which  conveys  these  stimuli  to  the  tympanic  membrane,  the  flexible 
partition  closing  the  canal  and  separating  it  from  the  middle  portion  of  the 
ear. 

The  Auricle. — The  outwardly  directed  external  surface  of  the  auricle 
is  irregularly  concave  and  presents  several  well-marked  depressions  and  ele- 
vations, which  depend,  for  the  most  part,  upon  the  corresponding  modelling 
of  tlie  underlying  cartilage. 


THE    EXTERNAL    EAR. 


363 


The  auricle  consists  of  integument  and  an  enclosed  plate  of  yellow  elastic 
cartilage,  continuous  with  that  of  the  meatus.  It  is  also  provided  with  several 
unimportant  ligaments  and  muscles.  The  lobule,  however,  contains  no  car- 
tilage, but  only  fibrous  tissue  and  fat  enclosed  within  the  integumentary  fold. 
The  skin  of  the  auricle  is  thin  and  closely  adherent  to  the  cartilage,  espe- 
cially on  the  outer  surface.  In  certain  parts  it  contains  fine  hairs  and 
sebaceous  and  sweat-glands.  The  hair-follicles  are  especially  abundant  over 
the  tragus,  antitragus  and  the  notch  lying  between  them,  the  hairs  guarding 


Bone 
Malleus 
Incus 


Inner  ear 

Semicircular  canal 


Internal  auditory  canal 
Auditory  nerve 


Endolymphatic  sac 


Cartilatre 


Fig.  412. — Diagram  sliowing  three  subdivisions  of  ear;  7/,  utricle;  j,  saccule  ;  blue  is  the  bony,  red  the 
membranous  labyrinth  of  the  internal  ear.     {Modified  from  Schwalbe.) 

the  entrance  into  the  external  auditory  canal,  known  as  tragi,  being  excep- 
tionally long.  The  sebaceous  glands  are  especially  well  developed  in  the 
cavity  of  the  concha. 

The  External  Auditory  Canal. — The  external  auditory  canal,  or 
meatus  acusticus,  leads  from  the  ca\'ity  of  the  concha  to  the  tympanic  mem- 
brane, which  closes  its  inner  extremity.  It  is  composed  of  an  outer  carti- 
lagino-membranous  (cartilaginous)  and  an  inner  bony  portion,  both  of  which, 
as  well  as  the  external  surface  of  the  tympanic  membrane,  are  lined  by  skin. 
The  cartilagino-membranous  part  contributes  something  more  than  one  third 
of  the  entire  length  of  the  canal,  and  is  a  continuation  of  the  cartilage  of  the 
auricle.      The  cartilage  of  the  canal,  histologically  of  the  elastic  type,  does 

but  is  deficient  at  its  upper  back  part,  where  it  is 
On  approaching  the  bony  portion,  this  deficiency 
marked  and  the   fibrous   tissue    correspondingly 


not  form  a  complete  tube, 
filled  in  by  fibrous  tissue, 
in   the    cartilage  is   more 
increased. 

The  skin  lining  the  outer  portion  of  the  canal  is  closely  attached  to  the 
underlying  cartilage  and  measures  about  1.5  mm.  in  thickness.  It  is  much 
thinner  within  the  bony  canal,  except  along  the  roof,  where  it  remains  rela- 
tively thick.  Over  the  outer  surface  of  the  tympanic  membrane,  the  skin  is 
reduced  to  a  very  delicate  and  smooth  investment,  covered  by  correspond- 
ingly attenuated  epidermis,   and  a  very  thin  layer  of  subcutaneous  tissue. 


364  NORMAL    HISTOLOGY. 

Numerous  fine  hairs  and  large  sebaceo2is  glands  occur  in  the  cartilaginous 
portion,  but  diminish  in  size  and  frequency  towards  the  bony  canal,  in  which 
thev  are  entirely  wanting.  Within  the  cartilaginous  meatus  and  along  the 
roof  of  the  bony  tube,  the  skin  is  closely  beset  with  the  large  coiled 
ceruminous  glands,  which  resemble  in  structure  modified  sweat-glands. 
Like  the  latter,  the  ceruminous  glands  consist  of  a  deeper  and  wider 
coiled  portion,  the  sca-etory  seg)ne7it,  and  a  long  narrow  excretory  duct, 
which  ends  in  most  cases  independently  on  the  free  surface  of    the    skin. 

^^^cjjjv,^^  '~'''^'^4^  Sebacfous  gland 


Cartilage 


4:-_ 


."^^^ ' 


y'  \  Carriage 

Corium 
Hair  follicle 


Fig.  413.— Section  of  skin  lining  cartilaginous  pari  of  external  auditory  canal.     X  3°' 

Sometimes,  particularly  in  the  very  young  child,  it  may  open  into  the 
duct  of  a  sebaceous  gland.  The  cuboidal  secreting  cells  contain  yellowish 
brown  pigment  particles  and  granules  resembling  fat.  The  ear-wax  or 
cerumen  is,  as  usually  found,  the  more  or  less  dried  mixture  of  the  secre- 
tions derived  from  both  varieties  of  glands,  together  with  discarded  squamous 
epidermal  cells. 

The  blood-vessels  distributed  to  the  interior  of  the  external  auditory 
canal  pierce  the  membranous  roof  of  the  cartilaginous  meatus  and  the  associated 
fibrous  tissue  and  form  capillary  networks  within  the  perichondrium  and  peri- 
osteum and,  within  the  skin,  around  the  glands  and  the  hair  follicles.  _  The 
deeper  veiJis  of  the  meatus  drain  the  bony  and  a  small  part  of  the  cartilaginous 
meatus.  The  lymphatics  of  the  external  auditory  canal  arise  from  a 
cutaneous  network,  from  which  trunks  pass  in  three  general  groups,  as  do 
those  of  the  auricle.      The  nerves  supplied  to  the  external  auditory  canal. 


THE   MIDDLE    EAR. 


365 


cleri\ed  from  tlie  auriculo-teniporal  l)ranch  of  the  trigeminus  and  from  the 
auricular  branch  of  the  pneunKjgastric,  are  chiefly  meduUatecl  and  sensurv. 
Sympathetic  fibres  end  in  connection  with  the  glands. 

THE   MIDDLE    EAR. 

The  middle  ear  includes  three  subdivisions:  the  tympanic  caviiy,  the 
Eustachian  tube,  and  the  mastoid  cells. 

It  is  an  irregular  air-chamber,  beginning  on  the  lateral  wall  of  the  naso- 
pharynx with  the  Eustachian  tube,  which  leads  upwards,  backwards  and  out- 
wards, for  about  one  inch  and  a  half  into  the  temporal  bone.  Opposite  the 
external  auditory  canal,  it  widens  into  the  tympanic  cayity  and  continues 
backwards  into  the  mastoid  cells. 

The  Tympanic  Cavity, — The  tympanic  cayity,  also  called  the  tym- 
panum, is  an  irregular  space  within  the  temporal  bone,  lying  between  the 


Stapedius 
muscle 


Posterioi 
crus  of~ 
stapes 


Vestibule~tTs- 

Foot-plate  '^^ 
of  stapes 


Basal  tuiii  ^ 
of  cochlea 


'€'>-     C 


Fig.  414. — Horizontal  section  through  human  middle  and  internal  ear;  the  malleus  is  attached  to 
tympanic  membrane  and  the  stapes  occludes  the  oval  window.  X  5'A-  (Preparation  bv  Dr. 
Ralph  Butler.) 

internal  ear  and  the  external  auditory  canal.  It  is  lined  with  mucous  mem- 
brane and  contains,  in  addition  to  the  air  which  enters  by  way  of  the  Eusta- 
chian tube,  the  chain  of  ear-ossicles. 

The  Membrana  Tympani. — The  tympanic  membrane  or  drum-head 
is  a  delicate  transparent  disk,  irregularly  oyal  or  ellipsoidal  in  outline  and 
concaye  on  its  outer  surface.  It  is  about  .10  mm.  thick,  except  at  the 
periphery,  where  it  is  thickened. 

Embedded  in  the  tympanic  membrane  is  the  handle  of  the  malleus 
(Fig.  415),  which  extends  from  a  point  near  its  middle,  upwards  and  forwards 


366 


NORMAL   HISTOLOGY. 


towards  its  periphery.  At  its  lower  end,  the  handle  of  the  malleus  is  flat- 
tened laterally  and  broadened  at  the  umbo,  which  corresponds  to  the 
deepest  part  of  the  concavity  of  the  membrane. 

The  tympanic  membrane  includes  three  main  layers:  (i)  the  middle  or 
fibrous  stratum  ;  (2)  \}i\&  external  ox  cutaneous  layer,  the  prolongation  of  the 
skin  lining  the  external  auditory  canal;  and  (3)  the  internal  or  mucous  mem- 
brane, a  continuation  of  the  mucous  membrane  clothing  other  parts  of  the 
tympanic  cavity. 

The  fibrous  layer,  the  membrane  proper,  represents  the  mesodermic 
portion  of  the  drum-head  and  consists  of  an  outer  stratum  of  radially  dis- 
posed fibres  which  diverge 
from  the  malleus  towards  the 
periphery  of  the  membrane, 
and  an  inner  stratum  of  circular 
fibres,  concentrically  arranged 
and  best  developed  near  the 
periphery  of  the  membrane 
but  absent  at  the  umbo.  The 
radiating  fibres,  on  the  con- 
trary, become  more  dense  at 
the  umbo.  Connective  tissue 
corpuscles,  spindle-shaped  in 
longitudinal  and  stellate  in 
cross-section,  lie  betv/een  the 
fibres  of  the  two  layers.  At 
the  periphery  of  the  membrane 
proper,  the  fibres,  especially 
those  of  the  radial  stratum,  are 
connected  with  those  of  a  ring 
of  thick  connective  tissue,  the 
anniihis  Jibrosus.  The  fibres 
of  the  annulus  run  in  various 
directions,  but  for  the  most  part 
radially,  that  is,  towards  the 
tympanic  membrane  proper 
(Fig.  416).  Rounded  con- 
nective cells  lie  between  these 
fibres. 

The  cutaneous  layer 
consists  of  a  thin  epidermal 
stratum,  composed  of  two  or 
three  rows  of  cells  and  a  deli- 
cate sheet  of  connective  tissue; 
neither  a  definite  corium  nor  papillae  are  present. 

The  mucous  membrane  covering  the  inner  surface  of  the  drum-head 
consists  of  a  scanty  layer  of  connective  tissue,  invested  with  a  sheet  of  large 
low  nonciliated  epithelial  cells. 

The  blood-vessels  of  the  tympanic  membrane  include  arteries  arranged 
as  an  outer  and  an  inner  set,  separated  by  the  membrane  proper.  Each  of 
these  sets  forms  a  plexus  of  vessels,  with  a  large  branch  extending  down- 
wards along  the  malleus-handle  and  another  around  the  periphery,  of  the 
membrane,  these  two  branches  being  connected  by  numerous  radiating  twigs. 
Perforating  vessels  connect  the   two   sets   of   arteries,  especially  along  the 


Head  of  malleus 
External  ligament 
Mem.  flaccida  or 
Shrapnell's  membrane 
"Prussak's  space 
Neck 

Short  process 

Chorda  tympani 

Tendon  of  tensor 

tympani 

Long  process  of 

malleus 


Cartilage 
Epidermis 
Membrana  propria 


Mucous  membrane 


Membrana  tympani 


Annulus  tendinosus 


Fk;.  415. —  Frontal  section  through  tympanic  membratie 
and  malleus,  showing  long  process  of  the  latter  embedded 
within  membrane.     X  8.     (Preparation  by  Dr.  Ralph  Butler.) 


THE   MIDDLE    EAR. 


367 


malleus-handle  and  at  the  periphery  of  the  membrane.  The  veins  are  most 
numerous  at  the  handle  of  the  malleus  and  periphery  of  the  membrane  and 
communicate  with  those  of  the  external  meatus  and  tympanic  cavity.  The 
lymphatics  are  arranged  similarly  to  the  blood-vessels  in  two  sets,  one 
under  the  skin  and  the  other  under  the  mucous  membrane,  which  commu- 
nicate freely  with  each  other.  The  nerves  supplying'  the  tympanic  mem- 
brane accompany,  for  the  most  part,  the  blood-vessels  and,  in  addition  to 
supplying  the  latter,  form  both  a  subcutaneous  and  a  submucous  plexus. 

The  auditory  ossicles  are  three  small  bones  that  form  a  chain 
extending-  across  the  upper  part  of  the  tympanum  from  the  tympanic  mem- 
brane to  the  labyrinth.      The  outermost  of  these,  the  malleiis  (hammer),  is 


Epi'nelium  cf  tMnpanic 
surface 


Circular  fibres 
Radial  fibres 


Tympanic  cavity •  A^it'f 


Mucous  membrane- 


Epidermis  of  drum-head 
Subepidermal  layer 
External  auditory  canal 

Epidermis  of  canal 


Corium  of  skin  lining 
canal 

Epidermis  passing  onto 
"drum-head 


Blood-vessels' 


'& 


1' 


Bone 


Radial  fibres  of  annulus 
fibrosus 


Fig.  416. — Section  through  attached  margin  of  tympanic  membrane,  showing  continuation  of  skin 
and  mucous  membrane  over  its  outer  and  inner  surfaces  respectively.  ■  75.  (Preparation  by  Dr. 
Ralph  Butler.) 


attached  to  the  tympanic  membrane;  the  innermost,  the  stapes  (stirrup),  is 
fixed  in  the  oval  window,  and  between  these  two  bones  and  connected  with 
both  of  them,  lies  the  third  link  in  the  chain,  the  incus  (anvil).  Their  sur- 
faces of  contact  are  covered  with  articular  cartilage  and  enclosed  to  form 
miniature  true  joints,  provided  with  fibrous  capsular  ligaments  and  synovial 
membranes.  The  bones  do  not  lie  exposed  within  the  tympanic  space,  but 
are  invested  with  folds  of  the  general  mucous  membrane  lining  the  cavity. 

The  Eustachian  Tube. — The  Eustachian  tube,  or  tuba  auditiva,  is  a 
canal,  partly  bony  and  partly  cartilaginous,  extending  from  the  lateral  wall  of 
the  naso-pharynx  backwards,  upwards,  and  outwards  to  the  anterior  part  of  the 
tymp^^num.  In  the  adult  it  measures  about  37  mm.  in  length,  of  which  approx- 
imately the  upper  third  (tympanic  portion)  belongs  to  the  bony  division, 
whilst  the  remainder  is  contributed  by  the  cartilaginous  division  of  the  tube 


368  NORMAL    HISTOLOGY. 

The  posterior  wall  of  the  pharyngeal  portion  is  formed  by  a  plate  of 
cartilage,  the  upper  margin  of  which  is  curled  outwards  upon  itself  to  form  a 
gutter,  which  appears  as  a  hook  on  transverse  section.  The  interval  between 
the  margins  of  this  cartilaginous  groove  is  filled  with  strong  fibrous  tissue, 
thus  completing  the  canal.  At  birth  the  cartilage  is  entirely  of  the  hyaline 
variety,  but  later  this  is  more  or  less  extensively  replaced  by  fibrocartilage, 
except  in  the  upper  part  where  the  hyaline  cartilage  persists. 

The  mucous  membrane  of  the  Eustachian  tube  lines  the  tube  through- 
out its  length,  but  differs  somewhat  in  the  cartilaginous  and  osseous  portions. 
That  in  the  former  resembles  the  mucous  membrane  of  the  naso-pharynx, 


Lateral  lamina       .  j  r;  \ 

~~, —    Cartilage  of  tube 

_^^ ,  ■'':.,,A^, 

Oblique  muscle-fibres      %'^'-'- '  V 


Lumen  of  tube  "  i\ 


vl** 


Glands 
Tensor  palati  ■,■...;  ■> 

'V;,„,.  .;-■■■' 

Levator  palati  - 

Fig.  417. — Section  across  cartilaginous  part  of  Eustachian  tube.     X  7. 

with  which  it  is  directly  continuous,  while  that  of  the  osseous  division  resem- 
bles, to  some  extent,  the  mucous  membrane  of  the  tympanic  cavity.  The 
epithelhmi  of  both  divisions  is  of  the  ciliated  stratified  columnar  type,  with 
some  goblet-cells.  The  cells  in  the  pharyngeal  division,  especially  in  the 
lower  part,  are  taller  than  those  of  the  tympanic  portion,  which  are  low 
cuboidal.  In  the  tympanic  portion  the  mucous  membrane  is  closely  united 
with  the  periosteum  and  contains  very  few  mucous  glands  and  little  or  no 
lymphoid  tissue.  In  the  cartilaginous  division,  on  the  contrary,  the  epithe- 
lium overlies  a  layer  of  such  tissue,  often  called  the  tubal  tonsiL  This 
tissue  is  especially  abundant  in  children,  and  beneath  it  are  found  numerous 
mucous  glands,  which  open  on  the  free  surface  of  the  tube.  These  glands 
extend  nearly  to  the  perichondrium  and  sometimes  can  be  traced  even 
through  the  fissures  in  the  cartilage  into  the  surrounding  connective  tissue. 
A  considerable  amount  of  adipose  tissue  often  occupies  the  submucosa  of 
the  lower  and  lateral  walls.  The  submucous  layer  is  well  developed  in  the 
cartilaginous  division  of  the  tube,  particularly  in  the  outer  membranous  wall. 
It  consists  of  loosely  arranged  fibro-elastic  tissue,  which  supports  the  mucous 
glands  and  the  larger  vessels  and  nerves. 


THE   INTERNAL    EAR.  369 

The  muscles  of  the  Eustachian  tube  are  the  levator  and  the  tensor 
palati,  which  He  beneath  and  to  the  inner  side  of  the  tube  and  to  its  outer 
side  respectively.  By  reason  of  the  intimate  attachment  which  both  muscles 
have  to  the  cartilage  of  the  tube,  since  both  take  partial  origin  from  this 
structure,  contraction  of  their  fibres  tends  to  draw  apart  the  walls  of  the 
canal ;  they  thus  serve  as  dilators. 

The  Mastoid  Cells. — The  tympanic  cavity  communicates  posteriorly, 
through  the  antrum,  with  a  variable  number  of  irregular  pneumatic  cavities, 
the  mastoid  cells,  so  called  because  the  majority  of  these  spaces  occupy  the 
mastoid  process.  Unlike  the  antrum,  these  cells  are  not  developed  at  birth. 
As  the  mastoid  process  develops,  the  original  diploetic  structure  is  usually 
more  or  less  replaced  by  larger  cavities  forming  the  pneumatic  type.  These 
spaces  are  filled  with  air  and  lined  by  a  very  thin  mucous  membrane,  which 
is  continuous  with  that  of  the  antrum  and  of  the  tympanic  cavity.  It  is 
closely  united  with  the  periosteum  and  possesses  a  layer  of  low  nonciliated 
squamous  epithelium. 

THE    INTERNAL    EAR. 

The  internal  ear  consists  essentially  of  a  highly  complex  membranous 
sac,  connected  with  the  peripheral  ramifications  of  the  auditory  nerve,  and 
a  bony  capsule,  which  encloses  all  parts  of  the  membranous  structure  and  is 
embedded  within  the  substance  of  the  petrous  portion  of  the  temporal  bone. 
These  two  parts,  known  respectively  as  the  membranous  and  the  bony  laby- 
rinth^ are  not  everywhere  in  close  apposition,  but  in  most  places  are  sepa- 
rated by  an  intervening  space  filled  with  a  fluid,  the  perilymph,  the  inner 
sac  lying  within  the  osseous  capsule  like  a  shrunken  cast  within  a  mould. 
The  membranous  labyrinth  is  hollow  and  everywhere  filled  with  a  fluid, 
called  the  endolyinph,  which  nowhere  gains  access  to  the  cavity  occupied  by 
the  perilymph.  The  internal  ear  is  closely  related  with  the  bottom  of  the 
internal  auditory  canal,  which  its  inner  wall  contributes,  on  the  one  side,  and 
with  the  inner  wall  of  the  tympanic  cavity  on  the  other.  Its  entire  length 
is  about  20  mm.,  and  its  long  axis  corresponds  closely  with  that  of  the 
pyramidal  or  petrous  portion  of  the  temporal  bone.  The  irregular  cavity  of 
the  bony  labyrinth  comprises  three  subdivisions:  a  middle  one,  the  vestibule; 
an  anterior  one,  the  cochlea;  and  a  posterior  one,  the  semicircular  canals. 
Both  the  front  and  hind  divisions  communicate  freely  with  the  vestibule,  but 
neither  communicates  with  the  membranous  labyrinth  nor,  in  the  recent  con- 
dition, with  the  tympanic  cavity.  Although  corresponding  in  its  general 
form  with  the  bony  compartments  of  the  cochlea  and  semicircular  canals,  the 
membranous  labyrinth  less  accurately  agrees  in  its  contour  with  the  bony 
vestibule,  since,  instead  of  presenting  a  single  cavity,  it  is  subdivided  into 
two  unequal  compartments,  known  as  the  saccule  and  the  utricle,  which  are 
lodged  within  the  bony  vestibule.  The  divisions  of  the  membranous  laby- 
rinth are,  therefore,  four,  which  from  before  backwards  are:  the  niembranous 
cochlea,  the  saccule,  the  utricle,  and  the  membranous  semicircular  canals. 

THE     OSSEOUS    LABYRINTH. 

The  vestibule,  the  middle  division  of  the  bony  labyrinth,  lies  between 
the  cochlea  in  front  and  the  semicircular  canals  behind  and  communicates 
freely  with  both.  It  is  an  irregularly  elliptical  cavity,  measuring  about  5 
mm.  from  before  backwards,  the  same  from  above  downwards,  and  from  3-4 
mm.  from  without  inwards.      The  lateral  (outer)  wall  separates  it  from  the 


370  NORMAL   HISTOLOGY. 

tympanic  cavity  and  contains  the  oval  window  with  the  foot-plate  of  the  stapes. 
The  margin  of  the  window  and  the  foot-plate  are  covered  with  hyaline  carti- 
lage and  connected  by  fibro-elastic  tissue,  thus  preventing  the  escape  of  the 
perilymph  from  the  vestibule.  The  medial  (inner)  wall  of  the  vestibule 
presents  two  depressions  separated  by  a  ridge,  the  crista  vestibuli.  The 
anterior  and  smaller  of  these  depressions  is  the  spherical  recess  and  lodges  the 
saccule.  The  posterior  and  larger  depression  is  the  elliptical  recess.  The 
anterior  wall  of  the  vestibule  is  pierced  by  the  large  opening  leading  into 

the  scala  vestibuli  of  the  coch- 
superior  ampulla  ^^Superior  canal      |g^_      Posteriorly  the  vcstibule 

Common  crus    \       £ ^  directly  communicates  with  the 

semicircular  canals  by  five  round 

Lodges  utricle^  \JfiP!^^«^^"^i  Openings. 

Lodges  ^-^.    ><^  fi^rr^^^^^  External  ^hc  three  Dony  semicir- 


saccu  e    -~^^,„-,^^      -^  ^       ^    -siiw   „ — ,i„        cular    canals — the   supej'ior, 

Cochlea— 4    ^L»  X      ^ '  -g    .posterior      the  posterior,  and  the  horizontal 

"^'  "  '  — lie  behind  the  vestibule,  their 

disposition  being  such  that  the 
Posterior  ampulla  P^^^es  of  the  three  canals  cor- 

FiG.4i8.-Cast  of  right  bony  labyrinth,  mesial  aspect.     X  2.      respond   With    the   SldeS   of   the 

corner  of  a  cube.      Each  canal 

possesses  at  one  end  a  dilatation,  called  the  osseons  ampulla.  The  semi- 
circular canals  open  into  the  posterior  part  of  the  vestibule  by  five  apertures, 
the  undilated  ends  of  the  superior  and  posterior  canals  joining  to  form  a 
common  limb  (Fig.  418).  The  horizontal  canal  alone  communicates  with 
the  vestibule  by  two  independent  openings. 

The  vestibule  and  the  bony  semicircular  canals  are  lined  by  a  very  thin 
periosteum  composed  of  a  feltwork  of  a  resistent  fibrous  tissue,  containing 
pigmented  connective  tissue  cells.  Endothelium  everywhere  lines  the  peri- 
lymphatic space  between  the  membranous  and  osseous  canals,  covering  the 
free  inner  surface  of  the  periosteum,  the  fibrous  trabeculae,  and  the  outer  or 
perilymphatic  surface  of  this  part  of  the  membranous  labyrinth. 

The  bony  cochlea  constitutes  the  anterior  part  of  the  labyrinth  and 
appears  as  a  short  blunt  cone,  about  5  mm.  in  height,  whose  base  forms  the 
anterior  wall  of  the  inner  end  of  the  internal  auditory  meatus.  Its  apex  is 
directed  horizontally  outwards.  The  bony  cochlea  consists  essentially  of  a 
tapering  central  column,  the  modiolus,  around  which  the  bony  canal,  about 
30  mm.  long,  makes  something  more  than  two  and  a  half  spiral  turns,  the 
basal,  middle,  and  apical.  The  conical  modiolus  has  a  broad  concave  base, 
which  forms  part  of  the  base  of  the  cochlea,  and  a  small  apex,  which  extends 
nearly  to  the  apex  of  the  cochlea  or  cupola.  It  is  much  thicker  within  the 
lowest  turn  of  the  canal  than  above,  and  is  pierced  by  many  small  canals  for 
the  nerves  and  vessels  to  the  spiral  lamina.  The  axis  of  the  modiolus,  from 
base  to  apex,  is  traversed  by  the  central  canal,  while  a  more  peripherally 
situated  channel,  the  canalis  spiralis,  encircles  the  modiolus  and  contains  the 
spiral  ganglion  and  a  spiral  vein.  Projecting  at  a  right  angle  from  the 
modiolus  into  the  canal  of  the  bony  cochlea  is  a  thin  shelf  of  bone,  the 
lamina  spiralis  ossea,  which  is  made  up  of  two  delicate  bony  plates  between 
which  are  fine  canals  containing  the  branches  of  the  cochlear  nerve.  The 
partial  division  of  the  canal  of  the  bony  cochlea  effected  by  the  osseous 
spiral  lamina  is  completed  by  the  membranous  spiral  lamina,  which  stretches 
from  the  free  edge  of  the  osseous  lamina  to  the  outer  wall  of  the  canal  (Fig. 
422).     The  upper  division  of  the  canal  is  called  the  scala  vestibuli  and  com- 


THE   INTERNAL    EAR. 


371 


municates  with  the  vestibule,  whilst  the  lower  division,  the  scala  tympani, 
would  open  into  the  tympanic  cavity,  were  it  not  separated  from  that  space 
by  the  secondary  tympanic  membrane.     The  latter  is  a  thin   fibro-elastic 


Scala  vestibuli 
Scala  tvm 


Modiolus 


Internal  auditory 
canal 


Apex  of  modiolus 


Lamina  spiralis 
/' ossea 


Canalis  spiralis 
modioli 


Fig.  419. 


-Cochlea  and  bottom  of  internal  auditory  canal  exposed  by  vertical  section  ;  cochlea  rests  with 
its  base  downwards  and  apex  pointing  upwards.     )<  5. 


sheet,  covered  internally  with  endothelium  and  externally  with  epithelium. 
The  scalae  communicate  with  each  other  through  an  opening,  the  helico- 
trema,  at  the  apex  of  the  cochlea. 


THE    MEMBRANOUS    LABYRINTH. 

The  membranous  labyrinth  lies  within  the  bony  labyrinth,  which  it 
resembles  in  general  form.  This  agreement  is  least  marked  within  the  vesti- 
bule, since  here  the  single  division  of  the  bony  capsule  is  occupied  by  two 
compartments  of  the  membranous  sac,  the  utricle  and  the  saccule.  The 
membranous  labyrinth  comprises:  (i)  the  utricle  and  the  saccule,  which, 
with  the  ductus  endolymphaticns,  lie  within  the  vestibule;  (2)  the  three 
membranous  semicircular  canals  lodged  within  the  bony  semicircular  canals; 
and  (3)  the  membranous  cochlea  enclosed  within  the  bony  cochlea.  The 
membranous  labyrinth  is  attached,  especially  in  certain  places,  by  connective 
tissue  to  the  inner  wall  of  the  bony  capsule.  The  interval  between  the 
membranous  and  bony  labyrinths,  largest  in  the  scalae  tympani  and  vestibuli 
of  the  cochlea  and  in  the  vestibule,  constitutes  the  perilymphatic  space  and 
contains  a  modified  lymphatic  fluid,  the  perilymph.  The  fluid  within  the 
membranous  labyrinth,  the  endolymph,  can  pass  from  one  part  of  the  labyrinth 
to  another,  although  the  saccule  and  utricle  are  only  indirectly  connected 
through  a  narrow  channel,  the  djictus  endolymphaficns,  and  the  saccule  and 
cochlear  duct  communicate  by  means  of  a  small  tube,  the  canalis  renniens. 

Structure  of  the  Utricle,  Saccule  and  Semicircular  Canals. — 
The  walls  of  these  subdivisions  of  the  membranous  labyrinth  are  made  up 
of  {a)  an  outer  fibrous  cotmective  tissue  lamella  and  (b)  an  inner  epithelial 
lining,  the  latter  consisting  thrpughout  the  greater  part  of  its  extent  of  a 
single  layer  of  thin  flattened  polyhedral  cells.  Beneath  the  epithelium,  espe- 
cially in  the  region  of  the  maculae,  is  (r)  a  thin  almost  homogeneous  hyaline 
membrayie,  with  few  cells.  This  middle  layer  presents  in  places  on  its  inner 
surface  small  papillary  elevations  covered  by  epithelium.     On  the  concave 


372 


NORMAL    HISTOLOGY. 


side  of  each  of  the  semicircular  canals,  which  occupy  only  about  one  third 
of  the  lumina  of  the  bony  tubes,  is  a  strip,  the  raphe,  of  thickened  epithelium 
in  which  the  cells  become  low  cylindrical  in  type.  Over  the  regions  receiv- 
ing the  nerve-fibres,  the  macidcB  acusticcz  and  the  cristts  aaistioB,  the  epi- 
thelium undergoes  a  marked  alteration,  changing  from  the  indifferent  covering- 
cells  into  the  highly  specialized  neuroepithelium. 

The  maculae  acusticae  are  about  3  mm.  long  by  2  mm.  broad,  the 
macula  of  the  saccule  being  a  little  narrower  (1.5-1.6  mm.)  than  that  of  the 
utricle  (2  mm.  ).      At  the  margin  of  these  areas  the  cells  are  at  first  cuboidal, 


Fig.  420. — Membranous  labyrinth  of  five-months  foetus,  postero-mesial  aspect.  i,  2,  3,  posterior, 
horizontal,  and  superior  semicircular  canals  ;  4,  5,  6,  their  ampullae  ;  7,  common  crus  of  superior  and 
posterior  canals;  8,  g,  recess  and  macula  of  utricle  (10);  11,  saccule  and  its  macula  (12);  13,  ductus  endo- 
lymphalicus  ;  14,  utriculo-saccular  canal ;  15,  canalis  reuniens,  opening  at  16  into  cochlear  duct ;  17,  blind 
sac  of  cochlear  duct  (iSj;  19,  basilar  membrane;  20,  ligamentum  spirale ;  a,  facial  nerve;  b,  vestibular 
nerve;  c,  branch  of  vestibular  nerve  to  posterior  canal ;  d,  branches  of  cochlear  nerve  to  Corti's  organ. 
X  6.     {Retzius. ) 

next  low  columnar,  and  then  abruptly  increase  in  length,  until  they  measure 
from  30-35  !';  in  contrast  with  their  usual  height  of  from  3-4  //..  The 
acoustic  area  includes  two  kinds  of  elements,  the  sustentacular  or  fibre-cells 
and  the  hair-cells.  The  sustentacular  cells  are  long,  rather  narrow,  irregularly 
cylindrical  elements  and  extend  the  entire  thickness  of  the  epithelial  layer, 
resting  upon  a  well-developed  basement-membrane  by  their  expanded  or 
divided  basal  processes.  They  present  a  swelhng  enclosing  an  oval  nucleus 
and  terminate  at  the  surface  in  a  cuticular  zone.  The  cylindrical  hair-cells 
are  broader  but  shorter  than  the  sustentacular  cells,  and  reach  from  the  free 
surface  only  as  far  as  the  middle  of  the  epithelial  layer,  where  each  cell 
terminates  usually  in  a  rounded  or  somewhat  swollen  end  containing  a  spher- 
ical nucleus.  The  central  end,  next  to  the  free  surface,  exhibits  a  differen- 
tiation into  a  cuticular  zone,  similar  to  that  covering  the  inner  ends  of  the 
sustentacular  elements.  From  the  free  border  of  each  hair-cell,  a  stif?  robust 
hair  (20-25  p.  long)  projects  into  the  endolymph.  This  conical  process, 
however,  is  resolvable  into  a  number  of  agglutinated  finer  hairs  or  rods. 

The  free  surface  of  the  neuroepithelium  within  the  saccule  and  the  utri- 
cle is  covered  by  a  remarkable  structure,  the  so-called  otolith  membrane. 
This  consists  of  a  gelatinous  membrane  in  which  are  embedded  numberless 
small  crystalline  bodies,  the  otoliths  or  ear-stones.  Between  it  and  the  cutic- 
ular zone  is  a  space  filled  with  endolymph,  through  which  the  hair-cells  pass 


THE    INTERNAL    EAR. 


373 


to  the  otolith  membrane.  The  otoUths,  also  called  otoconia,  are  minute 
crystals,  usually  hexagonal  in  form,  with  slightly  rounded  angles,  and  from 
9—1 1  II.  in  length.  They  are  composed  of  calcium  carbonate  with  an  organic 
basis. 

On  reaching  the  maculae  the  nerve-fibres  form  a  subepithelial  plexus, 
from  which  fine  bundles  of  fibres  pass  towards  the  free  surface.  The  fibres 
usually  lose  their  medullary  substance  in  passing  through  the  basement  mem- 


Trabeculffi 


Perilyiuphatic 
space 


Membranous 
-canal 


-  Trabeculae 


Bony  wall 


Fig.  421. — Transverse  section  of  superior  semicircular  canal,  showing  relations  of  membranous  to  bone 

tube.     X  35- 

brane  and  enter  the  epithelium  as  naked  axis-cylinders.  Passing  between 
the  sustentacular  cells  to  about  the  middle  of  the  epithelium,  they  break  up 
into  fine  fibrillar,  which  embrace  the  deeper  ends  of  the  hair-cells  and  give 
off  fine  threads  that  pass,  as  free  axis-cylinders,  between  the  cells  to  higher 
levels. 

The  crista  acustica  and  the  adjoining  planum  semilunatum  are 
covered  with  neuroepithelium  simila:r  to  that  of  the  maculae.  The  hairs  of 
the  hair-cells,  however,  are  longer  and  converge  to  and  are  embedded  within 
a  peculiar  dome-like  structure,  known  as  the  cupola,  which  probably  does 
not  exist  during  life,  but  is  an  artefact  formed  by  coagulation  of  the  fluid  in 
which  the  ends  of  the  hairs  are  bathed.  Otoliths  are  probably  not  present  in 
the  cristae  acusticce. 

The  Cochlear  Duct. — The  membranous  cochlea  or  ditctus  cochlearis 
lies  within  the  bony  cochlea  and  like  it  includes  from  two  and  one-half  to 
two  and  three-quarter  turns,  named  respectively  the  basal,  middle,  and  apical, 
the  latter  being  three  fourths  of  a  turn  at  the  apex  of  the  cochlea.  The 
tapering  tube  of  the  bony  cochlea,  winding  spirally  around  the  modiolus,  is 
subdivided  into  three  compartments  by  the  osseous  spiral  lamina  and  two 
membranes,  namely,  the  membranous  spiral  lamina  and  Reissner's  mem- 
brane.     The  membranous  spiral  lamina  or  basilar  membrane  extends  from 


374 


NORMAL   HISTOLOGY. 


the  free  border  of  the  lamina  spiralis  ossea  to  the  outer  wall  of  the  cochlea, 
where  it  is  connected  to  an  inward  bulging  of  the  periosteum  and  subperios- 
teal tissue,  called  the  spiral  ligament.  The  lower  of  the  two  tubes  thus 
formed  is  the  scala  tympani  and  communicates,  in  the  macerated  skull,  with 
the  tympanum  through  the  round  window.  The  upper  tube  is  subdivided 
into  two  compartments  by  an  exceedingly  delicate  partition,  known  as  Reiss- 
ner'  s  me^nbrane,  which  extends  from  the  upper  surface  of  the  osseous  lamina 
near  its  outer  end,  obliquely  upwards  and  outwards,  to  the  external  wall  of 
the  cochlea.  The  compartment  above  this  membrane  is  the  scala  vestibidi 
and  communicates  with  the  perilymphatic  space  of  the  vestibule.  The  scalae 
tympani  and  vestibuli  communicate  only  at  the  apex  of  the  cochlea  through 
the  helicotrema.      They  contain  perilymph  and  are  brought  into  relation  with 


Corti's  membrane 


Organ  of  Corti 


Ganglion  spirale 


Scala  vestibuli 


Ductus 
cochlearis 


-Scala 
tympani 


Basilar  membrane 


Ganglion  spiral 


L   ---^--7--^     ^-  ■"-■-'■, 


Cochlear  nerve  in  internal  auditory  cana] 


Modiolus 

Fig.  422. — Section  of  human  cochlea  passing  through  axis  of  modiolus.    X  12. 

the  subarachnoid  space  through  the  aquaeductus  cochleae.  They  are  lined 
by  a  delicate  fibrous  periosteum,  usually  covered  on  the  surface  which  is  in 
contact  with  the  enclosed  perilymph  by  a  single  layer  of  endothelial  plates. 
In  some  localities,  however,  as  on  the  tympanic  surface  of  the  basilar  mem- 
brane, the  lining  cells  retain  their  primitive  mesodermic  character  and  never 
become  fully  differentiated  into  endothelium. 

The  third  compartment,  the  ductus  cochlearis,  is  triangular  on  cross- 
section  (Fig.  423),  except  at  its  ends,  and  bounded  by  Reissner's  membrane 
above,  by  the  basilar  membrane  and  a  part  of  the  osseous  spiral  lamina 
below,  and  by  the  outer  wall  of  the  bony  cochlea  externally.  Save  for  the 
narrow  channel,  the  canalis  reuniens,  by  which  it  communicates  with  the 
saccule,  the  cochlear  duct  is  a  closed  tube  and  contains  endolymph.  It 
begins  below  as  a  blind  extremity,  the  ccBcitm  vestibulare^  lodged  within  the 
recessus  cochlearis  of  the  vestibule  and,  after  making  two  and  three-quarter 
turns  through  the  cochlea,  ends  above  at  the  cupola  of  the  cochlea  in  a 
second  blind  extremity,  the  ccecum  ciipiclare,  which,  is  attached  to  the  cupola 
and  forms  a  part  of  the  boundary  of  the  helicotrema. 

Architecture  and  Structure  of  the  Cochlear  Duct. — Reissner's  membrane  or 
membrana  vestibularis,  the  delicate  partition  separating  the  cochlear  duct  from  the  scala 
vestibuli,  begins  on  the  upper  surface  of  the  lamina  spiralis,  about  .2  mm.  medial  to 
the  free  edge  of  the  bony  shelf,  and  extends  at  an  angle  of  from  40-45°  with  the  lamina 


THE   INTERNAL   EAR. 


375 


spiralis  ossea  to  the  outer  wall  of  the  cochlea,  where  it  is  attached  to  the  periosteum. 
Notwithstanding  its  excessive  thinness  (.3/^),  it  consists  of  three  layers:  (a)  a  very 
delicate  middle  stratum  of  connective  tissue,  (d)  the  endothelium  covering  the  vestibu- 
lar side,  and  (r)  the  epithelium  derived  from  the  ectodermic  cochlear  duct.  It  also 
contains  sparingly  distributed  capillary  blood-vessels. 

The  outer  wall  of  the  cochlear  duct  (Fig.  423)  is  bounded  by  a  part  of  a  thick- 
ened crescentic  cushion  of  connective  tissue,  whose  convex  surface  is  closely  united 
with  the  bony  wall  and  whose  generally  concave  surface  looks  towards  the  cochlear 
duct.  This  structure,  the  ligamentum  spirale,  extends  slightly  above  the  attachment 
of  Reissner's  membrane  and  to  a  greater  distance  below  the  attachment  of  the  basilar 
membrane,  thus  forming  part  of  the  outer  walls  of  the  scalae  vestibuli  and  tympani. 
At  its  junction  with  the  basilar  membrane  it  presents  a  marked  projection,  the  crista 
basilaris,  while  a  very  slight  elevation  marks  the  point  of  attachment  of  the  membrane 


Mil 

psule  &4jfff  , 
chleafW?    / 


Bony  ca 

of  cochlea/-  Mr 

m 


Spiral  ligament 


Crista  basilaris 


Basila 
membrane 


■...J^M 


Vas  spiralis 


Fig.  423. — Part  of  section  ot  human  cochlea,  showing  cochlear  duct  with   Corti's  organ.     X  9°. 
(Preparation  by  Dr.  Ralph  Butler.) 

of  Reissner.  The  part  of  this  ligament  lying  between  these  projections  corresponds 
to  the  outer  wall  of  the  cochlear  duct.  Its  concave  free  inner  surface  is  broken  by  a 
third  elevation,  the  prominentia  spiralis,  or  accessoty  spiral  ligament,  distinguished 
usually  by  the  presence  of  one  large  {vas  prominetis)  or  several  small  blood-vessels. 
The  lower  and  smaller  of  these  two  divisions  of  the  outer  wall  is  called  the  sulcus 
spiralis  externus  and  is  lined  by  cuboidal  epithelium,  while  the  larger  upper  division 
is  occupied  by  a  peculiar  vascular  structure,  the  stria  vascularis,  which  contains  capil- 
lary blood-vessels  within  an  epithelial  structure.  Its  surface  is  covered  with  pigmented 
irregular  polygonal  epithelial  cells,  and  its  deeper  strata  consist  of  cells  which,  espe- 
cially in  the  superficial  layers,  resemble  the  surface  epithelium,  but  in  the  deeper  layers 
assume  more  and  more  the  character  of  connective  tissue.  Over  the  prominentia 
spiralis  the  cells  become  flat  and  polyhedral. 

The  ligamentum  spirale  is  composed  of  a  peculiar  connective  tissue,  rich  in  cells 
and  blood-vessels.  Its  thin  outer  layer  forms  the  periosteum  and  is  denser  than  the 
adjacent  loose  connective  tissue.  The  latter  is  broadest  opposite  the  scala  tympani, 
where  its  fibres  converge  towards  the  crista  basilaris.  Opposite  the  outer  wall  of  the 
cochlear  duct  it  again  becomes  more  compact  and  is  rich  in  cells  and  blood-vessels. 
An  internal  layer  extending  from  near  the  prominentia  spiralis  to  the  basilar  membrane 
consists  of  a  hyaline  noncellular  tissue.  Some  authors  claim  to  have  found  smooth 
muscle-fibres  in  the  ligamentum  spirale. 


376  NORMAL   HISTOLOGY. 

The  tympanic  wall  or  floor  of  the  cochlear  duct  (Fig.  423)  comprises  the  basilar 
membrane,  e.xtending  from  the  basilar  crest  to  the  outer  end  of  the  bony  spiral  lamina, 
and  the  limbus  -lauiincs  spiralis,  which  includes  this  wall  from  the  attachment  of  Reiss- 
ner's  membrane  to  the  end  of  the  bony  lamina.  The  limbus  or  crista  spiralis  is  a 
thick  mass  of  connective  tissue  upon  the  upper  surface  of  the  outer  end  of  the  osseous 
lamina  spiralis.  Its  outer  extremity  is  deeply  grooved  to  form  a  gutter,  the  sulcus 
spiralis  intermts,  the  projections  of  the  limbus  above  and  below  the  sulcus  forming 
respectively  its  superior  (vestibular)  and  inferior  (tympanic)  labia.  The  upper  sur- 
face of  the  limbus  is  mariced  by  clefts  and  furrows  which  are  most  conspicuous  near 
the  outer  margin  of  the  upper  lip,  where  the  irregular  projections  between  the  furrows 
form  the  so-called  auditory  teeth,  because  of  their  fancied  resemblance  to  incisor  teeth. 
The  lower  lip  is  continuous  externally  with  the  basilar  membrane  and  is  perforated 
near  its  outer  end.  by  some  4000  apertures  {foramina  nervosa)  transmitting  minute 
branches  of  the  cochlear  nerve.  The  epithelium  covering  the  elevated  portions  of  the 
limbus,  including  the  auditory  teeth,  is  of  the  flat  polyhedral  variety,  the  intervening 
furrows  and  clefts  being  lined  by  columnar  cells.  The  epithelium  of  the  sulcus  spiralis 
consists  of  a  single  layer  of  low  cuboidal  or  flattened  cells,  continuous  with  the  epithe- 
lium of  the  auditory  teeth  above  and  with  the  highly  specialized  elements  of  Corti's 
organ  below. 

The  basilar  membrane  consists  of  a  median  (inner)  and  a  lateral  (outer)  part. 
The  former,  known  as  the  zona  arcuata,  is  thin  and  supports  the  modified  neuroepi- 
thelium  constituting  the  organ  of  Corti ;  the  outer  part,  named  the  zonapectiiiata,  is  the 
thicker  division  and  lies  external  to  the  foot-plates  of  the  outer  rods  of  Corti.  The 
basilar  membrane  is  made  up  of  three  distinct  layers — the  epithelium,  the  substantia 
propria,  and  the  tympanic  lamella.  The  substantia  propria  is  formed  of  an  almost 
homogeneous  connective  tissue  with  a  few  nuclei  and  fine  fibres,  which  radiate  towards 
the  outer  edge  of  the  spiral  lamina.  The  fibres  of  the  zona  arcuata  are  very  fine  and 
interwoven,  appearing  to  be  an  extension  of  those  of  the  lower  lip  of  the  limbus,  whilst 
straight  and  more  distinct  fibres  stretch  from  the  outer  rods  of  Corti  to  the  spiral  liga- 
ment and  constitute  the  so-called  auditory  strings.  According  to  the  estimate  of 
Retzius,  there  are  24,000  of  these  special  fibres.  Their  length  increases  from  the  base 
towards  the  apex  of  the  cochlea,  in  agreement  with  the  corresponding  increase  in 
breadth  of  the  basilar  membrane.  The  tympanic  lamella  contains  numbers  of  fusi- 
form cells  of  immature  character  interspersed  with  fibres.  In  this  location  the  differ- 
entiation of  the  mesodermic  cells  lining  the  tympanic  canal  has  never  advanced  to  the 
production  of  typical  endothelial  plates,  the  free  surface  of  the  lamella  being  invested 
by  the  short  fusiform  cells  alone.  The  inner  zone  of  this  layer  contains  capillaries 
which  empty  into  one,  or  sometimes  two,  veins,  frequently  seen  under  the  tunnel  of 
Corti  and  known  as  the  vas  spirale.  The  epithelium  covering  the  inner  zone  of  the 
basilar  membrane  forms  the  organ  of  Corti,  the  highest  example  of  specialization  as 
neuroepithelium. 

The  Organ  of  Corti. — The  organ  of  Corti,  or  organon  spirale,  consists  in  a  general 
way  of  a  series  of  epithelial  arches  formed  by  the  interlocking  of  the  upper  ends  of  con- 
verging and  greatly  modified  epithelial  cells,  the  pillars  or  rods  of  Corti,  upon  the 
inner  and  outer  sides  of  which  rest  groups  of  neuroepithelial  elements — the  auditory 
and  the  sustentacular  cells.  The  triangular 'space  included  between  the  converging 
pillars  of  Corti  above  and  the  basilar  membrane  below  constitutes  the  tunnel  of  Corti, 
which  is,  therefore,  only  an  intercellular  space  of  unusual  size.  It  contains  probably 
a  soft  semifluid  intercellular  substance  serving  to  support  the  nerve-fibrils  traversing 
the  space  (Fig  424) .  The  pillars  or  rods  of  Corti,  examined  in  detail,  prove  to  be 
composed  of  two  parts,  the  denser  substance  of  the  pillar  proper,  and  a  thin,  imperfect 
protoi^lasmic  envelope,  which  presents  a  triangular  thickening  at  the  base  directed  tow- 
ards the  cavity  of  the  tunnel.  Each  pillar  possesses  a  slender  slightly  sigmoid,  longi- 
tudinally striated  body,  whose  upper  end  terminates  in  a  triangular  head,  and  whose 
lower  extremity  expands  into  the/bc/  resting  upon  the  basilar  membrane.  The  inner 
pillar  is  shorter,  more  perpendicular  and  less  curved  than  the  outer  ;  its  head  exhibits 
a  single  or  double  concave  articular  facet  for  the  reception  of  the  corresponding  con- 
vex surface  of  the  head  of  the  outer  rod.  The  cuticular  substance  of  both  pillars  ad- 
joining the  articular  surfaces  is  distinguished  by  a  circumscribed,  seemingly  homoge- 


THE    INTERNAL    EAR. 


377 


neous  oval  area  of  different  nature.  The  upper  straight  border  of  the  head  of  both 
pillars  is  prolonged  outwardl)-  into  a  thin  process  or  head-plate,  that  of  the  inner  lying 
uppermost  and  covering  over  the  head  and  inner  part  of  the  plate  of  the  outer  pillar. 
The  head-plate  of  the  latter  is  longer  and  projects  beyond  the  termination  of  the  plate 
of  the  inner  rod  as  the  phalangeal  process,  which  unites  with  the  adjacent  phalanges 
of  the  cells  of  Deiters  to  form  the  inembrana  reticularis.  The  inner  pillars  of  Corti 
are  more  numerous,  but  narrower  than  the  outer  elements,  from  which  arrangement 
it  follows  that  the  broader  outer  rods  articulate  with  two  and  sometimes  three  of  the 
inner  pillars,  the  number  of  the  latter  in  man  being  estimated  by  Retzius  at  5600,  as 
against  3S50  of  the  outer  rods. 

Immediately  medial  to  the  arch  of  Corti,  resting  upon  the  inner  rods,  a  single  row 
of  specialized  epithelial  elements  e.xtends  as  the  inner  auditory  or  hair-cells.  These 
elements,  little  more  than  half  the  thickness  of  the  epithelial  layer  in  length,  possess  a 
columnar  body  containing  an  oval  nucleus.  The  outer  somewhat  constricted  end  of 
each  hair-cell  is  limited  by  a  sharply  defined  cuticular  zone,  from  the  free  surface  of 
which  project,  in  man,  some  twenty-five  rods  or  hairs.     The  inner  hair-cells  are  less 


Nuel's  space 


Inner  hair-cells 


Outer  hair  cells 
Hensen  s  cells 


Cells  of  _^ 
Claudius 


Membrana  tectoria 


^fE)  tj^'5_X-_-l — spiralis 


''"^'Si---,  '  ljM^sr=^fC^^'^^'l 


Cells  of - 

Deiters        ~^^''«  «.<^  « 
Outer  pilUr 


--^>^^^<'^     Cochleai 


Fig. 


424. — Section  showing:  details  of  Corti's  organ  from  human  cochlea;   section  is  slightly  oblique, 
hence  width  is  somewhat  exaggerated.     X  375.     (Preparation  by  Dr.  Kalph  Butler.) 


numerous  (according  to  Retzius  about  3500) ,  as  well  as  shorter  and  broader,  than  the 
corresponding  outer  elements.  Their  relation  to  the  inner  rods  of  Corti  is  such,  that 
to  every  three  rods  two  hair-cells  are  applied.  The  inner  sustentacular  cells  extend 
throughout  the  thickness  of  the  epithelial  layer  and  exhibit  a  slightly  imbricated  ar- 
rangement as  they  pass  over  the  sides  of  Corti's  organ  to  become  continuous  with  the 
lower  cells  of  the  sulcus  spiralis. 

The  cells  covering  the  basilar  membrane  from  the  outer  pillar  to  the  basilar  crest 
comprise  three  groups  :  {a)  those  composing  the  outer  part  of  Corti's  organ,  including 
the  outer  hair-cells  and  cells  of  Deiters;  (b)  the  outer  supporting  cells,  or  cells  of 
Heiisen  ;  [c)  and  the  low  cuboidal  elements,  the  cells  of  Claudius,  investing  the  outer- 
most part  of  the  basilar  membrane. 

The  outer  auditory  or  hair-cells  are  about  five  times  more  numerous  (approxi- 
mately 18,0-jO  according  to  Waldeyer)  than  the  corresponding  inner  elements,  and  in 
man  and  apes  are  disposed  in  three  or  four  rows.  They  alternaie  with  the  peculiar 
end-plates  or  "  phalanges"  of  Deiters'  cells,  which  separate  the  ends  of  the  hair-cells 
and  join  to  f(jrm  a  cuticular  meshwork,  the  menibrana  reticularis,  through  the  open- 
ings of  which  the  hair-cells  reach  the  free  surface.  The  inner  row  of  these  cells  lies 
directly  upon  the  outer  rods  of  Corti,  so  placed  that  each  cell,  as  a  rule,  rests  upon 
two  rods.  The  cells  of  the  second  row,  however,  are  so  disposed  that  each  cell  lies 
opposite  a  single  rod,  whilst  the  third  layer  repeats  the  arrangement  of  the  first.  In 
consequence  of  this  grouping,  these  elements,  in  conjiuiction  with  the  phalanges, 
appear  in  surface  views  like  a  checker-board  mosaic,  in  which  the  oval  free  ends  of 
the  auditory  cells  are  included  between  the  peculiar  compressed  and  indented  octag- 


378 


NORMAL   HISTOLOGY. 


onal  areas  of  the  end-plates  of  Deiters'  cells  (Fig.  425).  The  outer  hair-cells  are  cylin- 
drical in  their  general  form,  terminating  about  the  middle  of  the  epithelial  layer  in 
slightly  expanded  rounded  ends,  near  which  the  spherical  nuclei  are  situated.  The 
outer  sharply  defined  ends  of  the  cells  are  distinguished  by  a  cuticular  border  support- 
ing about  twenty-five  rigid  auditory  rods  or  hairs  which  project  beyond  the  level  of 
the  membrana  reticularis.  The  deeper  end  of  each  outer  hair-cell  contains  a  dense 
yellowish  enclosure,  known  as  the  body  of  Retzius,  which  is  triangular  when  seen  in 
profile.     The  bodies  are  absent  in  the  inner  hair-cells. 

The  cells  of  Deiters  have  much  in  common  with  the  rods  of  Corti,  like  these  being 
specialized  sustentacular  epithelial  cells  which  extend  the  entire  thickness  of  the  epi- 
thelial stratum  to  terminate  in  the  peculiar  end-plates  or  phalanges.  It  follows,  that 
while  the  free  surface  of  Corti' s  organ  is  composed  of  both  auditory  and  sustentacular 


Cells  of  Hensen 


'—  Deiters'  cells 


Outer  hair-cells 


Plate-like  processes  ol 

Outer  pillar  cells 
Inner  hair-cells 


Fig.  425. — Corti's  organ  viewed  from  above,  showing  mosaic  formed  by  pillars  and  Deiters'  cells ;  outer 
ends  of  auditory  cells  occupy  meshes  of  cuticular  network.    {Retzius.) 


cells,  the  elements  resting  upon  the  basilar  membrane  are  of  one  kind  alone — the  cells 
of  Deiters.  The  bodies  of  the  latter  consist  of  two  parts,  the  elongated  cylindrical 
chief  portio7i  of  the  cell,  containing  the  spherical  nucleus  and  resting  upon  the  basilar 
membrane,  and  the  greatly  attenuated  pyramidal  phalangeal  process.  A  system  of 
communicating  intercellular  clefts,  the  spaces  of  Nuel,  lie  between  the  auditory  and 
supporting  cells  ;  like  the  tunnel  of  Corti,  these  spaces  are  occupied  by  a  semifluid  in- 
tercellular substance.  The  cells  of  Deiters  are  arranged,  as  a  rule,  in  three  rows,  al- 
though in  places  within  the  upper  turns  four  or  even  five  alternating  rows  are  some- 
times found.  Each  cell  contains  a  fine  filament,  the  fibre  of  Retziits,  which  begins 
near  the  middle  of  the  base  with  a  conical  expansion  and  extends  through  the  cell- 
body  to  the  apex  of  the  phalangeal  process. 

The  membrana  tectoria  or  Corti's  membrane  stretches  laterally  from  the  upper  lip 
of  the  limbus,  above  the  sulcus  spiralis  and  Corti's  organ,  as  far  as  the  last  row  of 
outer  hair-cells.  The  membrane  is  a  cuticular  production,  formed  originally  by  the 
cells  covering  the  region  of  the  auditory  teeth  and  the  spiral  sulcus.  Medially  it  rests 
upon  the  epithelial  cells,  but  farther  outward  it  becomes  separated  from  the  free  edge 
of  the  auditory  teeth  and  assumes  its  conspicuous  position  over  the  organ  of  Corti. 
The  membrane  seems  to  be  composed  of  fine  resistent  fibres,  held  together  by  an  in- 
terfibrillar  substance.  During  life  the  membrane  is  probably  soft  and  gelatinous,  and 
much  less  rigid  than  its  appearance  indicates  after  the  effect  of  reagents.  The  lower 
surface  of  the  free  portion  of  the  membrane,  opposite  the  inner  hair-cells,  is  modelled 
by  a  shallow  furrow,  which  indicates  the  position  of  a  spirally  arranged  band  known  as 
the  stripe  of  Hensen.  Like  the  basilar  membrane,  the  membrana  tectoria  increases  in 
width  from  the  base  towards  the  apex  of  the  cochlea, 


THE    INTERNAL    EAR.  379 

The  outer  sustentacular  cells  or  cells  of  Hensen  form  a  zone  immediately  external 
to  the  last  Deiters'  cells.  These  elements  resemble  the  inner  sustentacular  cells,  but 
differ  somewhat  in  form  and  arrangement.  In  consequence  of  their  oblique  position, 
the  bodies  are  not  only  greatly  elongated,  but  also  imbricated.  The  cells  of  Claudius 
are  the  direct  continuations  of  Hensen's  cells,  and  laterally  pass  uninterruptedly  into 
the  low  columnar  elements  covering  the  remaining  part  of  the  basilar  membrane. 
They  consist  of  a  single  row  of  cuboidal  cells  possessing  clear,  faintly  granular  proto- 
plasm and  spherical  nuclei. 

The  Nerves  of  the  Membranous  Labyrinth. — The  branches  of 
the  cochlear  division  of  the  auditory  nerve  enter  the  base  of  the  cochlea 
through  numerous  small  foramina,  those  destined  for  the  apical  turn  travers- 
ing the  central  canal  of  the  modiolus.  From  the  modiolus  a  series  of  stout 
lateral  branches  diverge  at  quite  regular  intervals  through  canals  which  com- 
municate with  the  peripheral  spiral  canal  within  the  base  of  the  bony  spiral 
lamina.  Within  the  peripheral  canal  the  nerve-fibres  join  numerous  aggre- 
gations of  bipolar  nerve-cells,  which  continue  along  the  spiral  canal  and  col- 
lectively constitute  the  ganglion  spirale.  From  these  cells  numerous  den- 
drites are  given  off,  which  pass  along  the  canals  within  the  spiral  lamina 
towards  its  margin,  the  twigs  meanwhile  subdividing  to  form  an  extensive 
plexus  contained  within  corresponding  channels  in  the  bone.  At  the  edge 
of  the  spiral  lamina,  bundles  of  fine  fibres  are  given  off,  which  enter  the  epi- 
thelial layer  close  to  the  inner  rod  of  Corti.  During  or  before  their  escape 
from  the  lamina,  the  nerve-fibres  lose  their  medullary  substance  and  proceed 
to  their  destination  as  fine  naked  axis-cylinders.  The  radiating  bundles  pass 
within  the  epithelium  to  the  mesial  side  of  the  base  of  the  inner  pillar ;  here 
they  divide  into  two  sets  of  fibrillae,  one,  the  mesial  spiral  fasciculus,  going 
to  the  inner  hair-cells  and  the  other,  the  lateral  spiral  fasciculus,  passing  be- 
tween the  inner  pillars  to  reach  the  tunnel  of  Corti.  Within  this  space 
fibrillae  are  given  off  which,  after  crossing  the  tunnel,  escape  between  the 
outer  rods  into  the  epithelium  lying  on  the  lateral  side  of  the  arch.  The 
further  course  of  the  fibrillse  seems  to  be  such  that  some  extend  between  the 
outer  pillar  of  Corti  and  the  first  rows  of  hair-cells,  whilst  succeeding  groups 
of  fibrillae  course  between  the  rows  of  Deiters'  cells  to  reach  the  remaining 
hair-cells.  The  relation  between  the  nerve-fibrils  and  the  auditory  cells  is 
in  all  cases  probably  close  contact  and  not  actual  junction. 

The  nerves  supplying  the  saccule,  utricle,  and  the  semicircular  canals 
are  all  fibres  from  the  vestibular  division  of  the  auditory  nerve.  They 
traverse  the  bony  labyrinth  through  canals  that  open  internally  at  certain 
areas,  the  maculte  cribroscs,  by  numerous  openings  in  close  relation  to  the 
specialized  areas  in  the  wall  of  the  membranous  labyrinth.  The  relations  of 
the  nerve  fibres  to  the  receptive  cells  of  these  maculae  and  cristae  have  been 
described  (page  373). 

Blood-Vessels  of  the  Membranous  Labyrinth. —  The  auditory 
artery,  a  branch  of  the  basilar,  after  entering  the  internal  auditory  meatus 
divides  into  three  branches,  (i)  The  vestibular  artery  accompanies  the 
utriculo-ampuUary  nerve  and  supplies  the  upper  part  of  the  vestibule,  includ- 
ing the  posterior  part  of  the  utricle  with  its  macula,  the  saccule  and  the  cristae 
of  the  upper  and  outer  ampullae  of  the  corresponding  semicircular  canals. 
(2)  The  cochlear  artery  pursues  a  spiral  course.  It  gives  off  three  branches, 
two  of  which  are  distributed  to  the  lower  turn  of  the  cochlea,  while  the 
third  supplies  the  middle  and  apical  turns.  (3)  The  vestibulo-cochlear 
artery  arises  either  from  the  cochlear  artery  or  independently  and  divides, 
within   the  spiral  lamina,   into  a  cochlear  and  a  vestibular  branch.      The 


38o  NORMAL    HISTOLOGY. 

cochlear  branch  is  distributed  to  the  lower  turn  of  the  cochlea  and  anasto- 
moses with  the  cochlear  artery  proper.  The  vestibular  branch  is  distributed 
to  the  lower  part  of  the  vestibule,  including  the  lower  part  of  the  saccule  and 
utricle,  to  the  crus  commune  and  part  of  the  semicircular  canals,  and  to  the 
lower  end  of  the  cochlea.  The  macula  of  the  saccule  receives  its  arterial 
supply  from  a  blood-vessel  which  usually  arises  from  the  common  stem  of 
the  vestibulo-cochlear  artery,  or,  more  rarely,  runs  independently  through 
the  whole  internal  meatus.  A  similar  origin  applies  to  the  artery  supplying 
the  nerve  of  the  posterior  ampulla.  In  the  base  of  the  spiral  lamina  the 
arteries  are  connected  by  capillar}^  loops  especially  in  the  lower  turn  of  the 
cochlea.  One  or  more  spiral  vessels  are  often  seen  under  the  tunnel  of  Corti 
within  the  tympanic  covering  of  the  basilar  membrane.  The  region  of  the 
stria  vascularis  and  prominentia  spiralis  is  especially  well  supplied  with 
blood-vessels.  Those  seen  in  the  scala  tympani  are  principally  veins,  while 
a  larger  number  of  arteries  are  found  in  the  scala  vestibuli.  The  blood- 
supply  of  the  lower  turn  of  the  cochlea  is  much  more  generous  than  that  of 
the  others. 

The  veins  from  the  cochlea  include :  ( i )  the  vein  of  the  vestibular  aque- 
duct, which  collects  the  blood  from  the  semicircular  canals;  (2)  the  vein  of 
the  cochlear  aqueduct,  which  collects  from  the  whole  cochlear  canal  through 
the  anterior,  posterior  and  middle  spiral  veins  and  from  most  of  the  vestibule 
through  the  anterior  and  posterior  vestibular  veins;  and  (3)  the  venous  plexus 
of  the  inner  auditory  canal,  which  receives  the  large  central  cochlear  vein. 

THE  NOSE. 

Although  only  a  small  part  of  the  nasal  chambers  is  occupied  by  the 
peripheral  olfactory  organ  in  man,  the  greater  part  forming  the  beginning  of 
the  respiratory  tract,  comparative  anatomy  and  embryology  establish  the 
primary  significance  of  the  nasal  groove  and  its  derivations  as  the  organ  of 
smell,  the  relation  of  the  nose  to  respiration  being  entirely  secondary.  The 
nose,  therefore,  is  appropriately  grouped  with  the  organs  of  special  sense. 

The  nose  is  conventionally  divided  into  two  portions:  the  external 
nose,  consisting  of  a  framework  of  bone,  cartilage  and  fibrous  tissue,  which 
insures  the  .maintenance  of  apertures  for  the  passage  of  air,  and  the  nasal 
chamber,  divided  by  a  septum  into  the  right  and  left  nasal fosscB  and  lined 
by  mucous  membrane.  The  junction  of  the  latter  with  the  skin  is  marked 
by  a  limiting  ridge,  the  livien  vestibuli,  on  the  inner  surface  of  the  vestibule, 
a' short  distance  above  the  nostril.  Apart  from  the  unusually  large  seba- 
ceous glands  surrounding  and  the  hairs  ivibrissce')  guarding  the  nares,  the 
external  nose  presents  little  of  especial  histological  interest.  The  description 
of  the  external  nose  and  of  the  complicated  modelling  of  the  nasal  fossae  fall 
within  the  province  of  gross  anatomy.  The  present  consideration  of  the 
nose,  therefore,  may  be  limited  to  the  mucous  membrane  Hning  the  nasal 
chamber. 

THE   NASAL   iMUCOUS    MEMBRANE. 

Beyond  the  limit  of  the  integument  clothing  the  vestibule,  the  nasal 
fossa  is  lined  bv  mucous  membrane  continuous  with  that  of  the  naso-pharynx 
through  the  choanae.  Since  in  addition  to  lining  the  tract  over  which  the 
respired  air  passes  the  nasal  mucous  membrane  contains  the  cells  receiving 
the  impressions  giving  rise  to  the  sense  of  smell,  it  is  appropriately  divided 
into  a  respiratory  and  an  olfactory  part. 


THE    NOSE. 


3«i 


The  Olfactory  Region. — The  highly  speciaHzed  regie  olfactoria  is 
quite  limited  in  extent  and  embraces  an  area  situated  over  the  upper  and 
adjoining-  part  of  the  middle  turbinate  and  the  corresponding  part  of  the 
septum.  In  fresh  preparations  the  olfactory  area  usually,  but  not  al\va}'s, 
can  be  approximately  mapped  out  by  the  yellowish  hue,  lighter  or  darker, 
that  distinguishes  it  from  the  respiratory  region  in  wliich  the  mucous 
membrane  exhibits  a  rosy  tint. 

The  epithelium  contains  two  chief  constituents — the  supporting  and 
the  olfactory  cells.  The  supporting  cells  are  tall  cylindrical  elements, 
about  60  ,a  in  height,  that  extend  the  entire  thickness  of  the  epithelium. 
Their  outer  and  broader  ends  are  of  uniform  width  and  contain  the  oval 
nuclei  which,  lying  approximately  at  the  same  line  and  staining  readily, 
form  a  deeply  colored  and  conspicuous  nuclear  stratum  at  some  distance 
beneath  the   free  margin.      Between  the  latter  and  the  row  of  nuclei,   the 


Outer  zone 

Nuclear  layer  of 
supporting  cells 

Olfactory 


Blood 


Bundle  of 

olfactory  nerves 


Fig.  426. — Section  of  olfactory  mucous  membrane.     X  300. 


epithelium  presents  a  clear  zone  devoid  of  nuclei.  The  inner  part  of 
the  supporting  cells  is  thinner  and  irregular  in  contour  and  often  terminates 
by  splitting  into  two  or  more  basal  processes  that  rest  upon  the  tunica 
propria.  Between  these  ends  lie  smaller  pyramidal  elements,  the  basal 
cells,  that  probably  represent  younger  and  supplementary  forms  of  the 
sustentacular  cells.  The  granular  protoplasm  of  the  basal  processes  often 
contains  pigment  particles. 

The  olfactory  cells,  the  perceptive  elements  receiving  the  smell- 
stimuli,  consist  of  a  fusiform  body,  lodging  a  spherical  nucleus  enclosed  by 
a  thin  envelope  of  cytoplasm,  and  two  attenuated  processes,  a  peripheral 
and  a  central.  The  olfactory  cells  are  in  fact  sensory  neurones  that  have 
retained  their  primitive  position  within  the  surface  epithelium,  as  in  many 
invertebrates,  instead  of  receding,  as  is  usual  in  the  higher  animals,  to  situa- 
tions more  remote  from  the  exterior.  The  slender  peripheral  process  of  the 
olfactory  cell,  which  corresponds  to  the  dendrite  of  the  neurone,  is  of  uniform 
thickness  and  ends  at  the  surface  in  a  small  hemispherical  knob  that  projects 


382 


NORMAL    HISTOLOGY. 


slightly  beyond  the  general  level  of  the  epithelium  and  bears  from  6-8  minute 
stifi  cilia,  the  olfactory  hairs.  Being  dependent  upon  the  position  of  the 
nuclei,  the  length  of  the  peripheral  processes  varies,  since  the  nuclei  occupy 
different  levels  within  the  epithelium  in  order  to  accommodate  their  great 

number.      The  central  processes  of  the  olfactory 

cells,  much  more  delicate  than  the  peripheral, 

are  directly  continued,    as   the   axis-cylinders, 

Olfactory  cell         into  the  subjacent  nonmedullated   nerve-fibres 

within  the  tunica  propria. 


^-^%^y-Supporting:  cell 


Nerve-fibre 


Fig.  427. — Section  of  olfactory  tnu- 
cous  membrane,  silver  preparation  ; 
two  olfactory  cells  are  seen,  one 
sending  nerve-fibre  towards  the 
brain.     X  335-     {Brunn.) 


Fig.  428. — Isolated  ele- 
ments of  epithelium  of  olfac- 
tory mucous  membrane;  a, 
olfactory  cells ;  6,  support- 
ing cells.    X  600.    (^Biunn.) 


from  which  they  pass 
through  the  cribriform 
plate  of  the  ethnoid  to 
enter  the  brain  and  end  in 
the  arborizations  within 
the  olfactory  glomeruli  of 
the  bulbus  olfactorius. 

The  tunica  propria 
is  differentiated  into  a  su- 
perficial and  a  deep  layer 
by  the  lymphoid  charac- 
ter of  the  stratum  directly 
beneath  the  epithelium. 
The  superficial  layer,  from 
15-20  p.  thick,  consists  of 
closely  packed  irregularly  round  cells,  resembling  lymphocytes,  and  meagre 
bundles  of  delicate  connective  tissue.  The  deep  layer,  on  the  other  hand, 
contains  robust  bundles  of  fibro-elastic  tissue  and  relatively  few  cells. 
A  distinct  merabrana  propria  is  wanting  within  the  olfactory  region.  , 

The  olfactory  glands,  ox  glands  of  Bowman ,  are  characteristic  of  the 
olfactory  region  and  probably  elaborate  a  specific  secretion.  They  open 
onto  the  free  surface  by  very  narrow  ducts  that  lead  into  saccular  fusiform 
dilatations,  the  ampiiUcB,  into  which  the  tubular  alveoli  pass.  The  ducts 
possess  an  independent  lining  of  flattened  cells,  that  extend  as  far  as  the 
surface  and  lie  between  the  surrounding  epithelial  elements.  The  dilatations 
are  clothed  with  flattened  or  low  cuboidal  cells,  which  are  replaced  by  those 
of  irregular  columnar  or  pyramidal  form,  often  pigmented,  within  the  tubular 
alveoli.  From  the  character  of  their  secretion,  the  glands  of  Bowman  are 
probably  to  be  reckoned  as  serous  and  not  mucous. 

The  Respiratory  Region. — The  mucous  membrane  lining  the  res- 
piratory region  differs  greatly  in  thickness  in  various  parts  of  the  nasal 
fossa.  In  situations  where  the  contained  cavernous  tissue  is  well  repre- 
sented, as  over  the  inferior  turbinate,  it  may  reach  a  thickness  of  several 
millimeters,  while  when  such  tissue  is  wanting,  as  on  the  lateral  wall,  it  is 
reduced  to  less  than  a  millimeter. 

The  epithelium  is  stratified  ciliated  columnar  in  type,  froin  50-70  t>- 
thick,  and  includes  the  tall  ciliated  surface  cells,  between  whose  inner  ends 
lie  the  irregularly  columnar  basal  cells.  Numerous  elements  exhibit  various 
stages  of  conversion  into  mucus-containing  goblet-cells.  The  current  produced 
by  the  cilia  is  towards  the  posterior  nares.  Intraepithelial  migratory  lympho- 
cytes are  also  common.  Beneath  the  epithelium  stretches  the  membrana 
propria,  that  varies  greatly  in  thickness;  although  in  certain  localities 
feebly  developed,  it  is  usually  well  marked  and  measures  from  2—10  p.  in 
thickness.      Under  pathological  conditions  its  thickness  may  increase  four- 


THE   NOSE. 


383 


fold  or  more.  In  many  places  the  membrana  propria  is  pierced  by  minute 
vertical  channels,  the  basal  canals,  in  which  connective  tissue  cells  and 
leucocyctes  are  found. 

The  tunica  propria  consists  of  interlacing  bundles  of  fibro-elastic  tissue 
which  are  most  compactly  disposed  towards  the  subjacent  periosteum.      The 

J:.- Ill  *.?,>•* 


Epithelium - 
Pit- 


Duct  of  glands 


Blood-vessel 


Glands 


Fig.  429. — Section  of  respiratory  mucous  membrane  covering'  nasal  septum.     X  75. 

looser  superficial  stratum  is  rich  in  cells  and  here  and  there  contains  aggre- 
gations of  lymphocytes  that  may  be  regarded  as  masses  of  adenoid  tissue. 
In  certain  parts  of  the  nasal 


Goblet-cell  in 
epithelium 


Tunica  propria 


Blood-vessel 


fossae,  the  stroma  of  the 
mucous  membrane  contains 
vascular  areas  composed  of 
numerous  intercommunicat- 
ing blood-spaces  that  confer 
the  character  of  a  true  caver- 
nous tiss.ue.  These  special- 
ized areas,  the  co7'pora  caver- 
nosa, as  they  are  called,  are 
especially  well  developed  over 
the  inferior  and  the  lower 
margin  and  posterior  extrem- 
ity of  the  middle  turbinate, 
and  less  so  over  the  posterior 
end  of  the  upper  turbinate. 
Where  typical,  they  occupy 
practically  the  entire  thickness 
of  the  mucous  membrane  from 
periosteum  to  epithelium,  th^  interlacunar  trabeculae  containing  the  glands 
and  blood-vessels  destined  for  the  subepithelial  stroma.  The  blood-sinuses 
include  a  superficial  reticular  zone  of  smaller  spaces  and  a  deeper  one  of 
larger  lacunae. 


L'®^^-'Glands 


Fig.  430.— Section  of  mucous  membrane  lining^  maxillary  sinus. 
X  2S0.     (Preparation  by  Dr.  J.  P.  Tunis.) 


384 


NORMAL    HISTOLOGY 


The  glands  of  the  respiratory  region  are  very  numerous,  although 
varying  in  size,  tubo-alveolar  in  form  and,  for  the  most  part,  mixed  mucous 
in  type.      The  chief  ducts  open  on  the  free  surface  by  minute  orifices  barely 


HiJ, 


m^^ 


^^/_AJL  Blood. 


Fig.  431  — Section  of  mucous  membrane  Iming 
frontal  sinus.  X  280.  (Preparation  by  Dr.  J.  P. 
Tunis.) 


Fig.  432. — Section  of  mucous  membrane  Tining 
sphenoidal  sinus.  X  2S0.  (Preparation  by  Dr.  J.  P. 
Tunis.) 


./ 


\ 


distinguishable  with  the  unaided  eye-  Their  deeper  ends  branch  irregularly 
into  tubes  that  bear  the  ovoid  terminal  alveoli.  The  latter  are  lined  with 
mucus-secreting  cells,  between  which  lie  the  crescentic  groups  of  serouscells 
that  stamp  the  glands  as  mixed  (Stohr).  In  exceptional  cases  exclusively 
serous  glands  are  also  encountered. 

The  nasal  fossae  communicate  with  a  number  of  remarkable  cavkies,  the 
accessory  air-spaces,  hollowed  out  within  the  surrounding  bones,   which 

are  filled  with  air  and  lined  by 
mucous  membrane  directly 
continuous  with  that  of  the 
meatuses.  These  pneumatic 
spaces  include  the  maxil- 
/arj',  the  frontal  and  the 
sphenoidal  smuses  and  the 
ethmoidal  air-cells^  all  paired 
and  within  the  corresponding 
bones.  The  mucous  mem- 
brane lining  these  spaces 
resembles  in  general  that  of 
the  adjoining  nasal  fossae, 
but  is  very  much  thinner. 
It  includes  a  stratified  cili- 
ated columnar  epithelium, 
invaded  by  numerous  lymph- 
oid cells,  a  delicate  basement 
membrane  and  a  tunica  pro- 
pria, poor  in  elastic  fibres 
and  inseparably  blended  with 
the  periosteum,  of  which,  in 
fact,  it  is  part.  Small  scat- 
tered mucous  glands  occur 
in  meagre  numbers  in  the 
maxillary  sinus,  being  most 
plentiful  in  the  vicinity  of  the  opening  into  the  nasal  fossa. 

Jacobson's  Organ. — Mention  should  be  made  of  the  rudimentary 
structure  {organoji  vomeronasale)  found  in  man,  almost  constantly  in  the 
new-born  child  and  frequently  in  the  adult,  as  a  representative  of  the  organ  of 


'-^—Mucous 
membrane 


i~;:^omerine 
\y   cartilage 


-jacobson's 
organ 


Fig.  433. — Part  of  frontal  section  through  nasal  fossae  of  kitten, 
showing  organs  of  Jacobsoii.     X  20. 


THE   TASTE-BUDS. 


385 


Jacobson  that  is  present,  in  varying  degrees  of  perfection,  in  all  amniotic 
vertebrates  (Peter).  In  many  animals  possessing  in  high  degree  the  sense 
of  smell,  the  organ  is  well  developed  and  functions,  serving  possibly  as  an 
accessory  and  outlying  surface  by  which  the  first  olfactory  impressions  are 
received  (Seydel). 

In  man  the  organ  is  represented  by  a  laterally  compressed  tubular 
diverticulum,  from  1.5-6  mm.  in  length,  that  passes  backwards  to  end 
blindly  beneath  the  mucous  membrane  on  each  side  of  the  nasal  septum. 
The  median  wall  of  the  diverticulum  is  clothed  with  tall  columnar  cells 
resembling  those  of  the  olfactory  region,  the  characteristic  olfactory  cells, 
however,  being  wanting.  The  epithelium  covering  of  the  lateral  wall  cor- 
responds to  that  of  the  respiratory  organ.  In  many  animals  possessing 
acute  olfactory  sense,  branches  of  the  olfactory  nerve  are  traceable  to 
Jacobson' s  organ  in  which  are  found  olfactory  cells. 

THE  TASTE-BUDS. 

In  the  description  of  the  tongue  and  its  papillae  (page  144),  reference  is 
made  to  the  presence  of  specialized  epithelial  structures,  the  taste-buds, 
that  serve  for  the  reception  of  gustatory  stimuli.  These  bodies  collectively 
constitute  the  peripheral  sense-organ  of  taste  and  as  such  will  be  here  con- 
sidered. 

As  implied  by  their  name,  the  taste-buds  or  calyc7ih'  g7istato7-n  ^lyq  irreg- 
ular ellipsoidal  or  conical  bodies,  sometimes  broadly  oval  but  more  often 


Lymphoid  area 

Posterior  limit  of 

papillary  area 

ife 

,  Circumvallate 

"^    papillae 

■~~-  Palatine  arch 

ir- 

~p>- Foliate  papillas 

Fungiform, 
surrounded  by 
filiform  papilla 

Fig.  434.— Part  of  dorsum  of  the  tongue,  showing  varieties  of  the  papillae ;  natural  size. 

slender  in  outline,  and  in  the  adult  measure  from  70-80  //  in  length  and 
about  half  as  much  in  breadth.  Since  they  lie  entirely  within  the  epithelium 
clothing  the  mucous  membrane,  the  necessary  access  to  the  interior  of  the 
buds  is  afforded  by  mirmte  pore-canafs,  each  of  which,  beginning  on  thefree 
surface  at  the  otder  taste-pore,  leads  through  the  intervening  layer  of  epithe- 
lium to  the  imier  pore  that  caps  the  subjacent  pole  of  the  bud.  By  means 
of  these  canals  the  sapid  substances  dissolved  in  the  fluids  of  the  mouth 
25 


386  NORMAL   HISTOLOGY. 

reach  and  impress  the  gustatory  cells  within  the  taste-buds.  Pore-canals  are 
not,  however,  invariably  present,  since,  as  pointed  out  by  Graberg,  certain 
taste-buds  remain  immature  and  retain  their  embryonal  form  and  relations, 
being  broad  and  conical  and  in  contact  with  the  free  surface.  In  such  buds 
the  gustatory  cells  are  few,  only  two  or  three,  and  so  superficially  placed  that 
a  distinct  canal  is  absent.  Occasionally  double  buds  are  encountered  in 
which  two  gustatory  bodies  are  implanted  by  a  common  base,  but  parti)'' 
retain  their  independence  in  having  separate  distal  poles,  each  pro^dded  with 
its  separate  taste-pore  and  canal. 

The  chief  position  of  the  taste-buds  is  within  the  epithelium  lining  the 
sides  of  the  annular  groove  on  the  circumvallate  papillae,  the  buds  being 
more  numerous  and  closely  placed  on  the  median  than  on  the  lateral  wall  of 
the  furrow.  Their  number  has  been  variously  estimated,  but  it  is  probable 
that  from  loo  to  150  represents  the  maximum  for  a  single  papilla.      The  local- 

X^^^S^^^S^^^"^^  Taste-bud 

Epithelium  /  '  ,  /  Annular  wall 


Gland-duct 


Central  part  of/^  Serous  gland 

papilla—^  I, -:,;,.  ,, 

connective  tissue  ^  IJ},-:'^-'"  -^ 

Muscle  fibres^^^'^^^SSSp^ft^^^' 

Fig.  435. — Section  across  circumvallate  papilla  from  tongue  of  child ;  the  taste-buds  are  seen  within  the 

epithelium.    X  45. 

ity  of  next  importance  numerically  is  the  papillae  foliatae  on  the  sides  of  the 
tongue  in  the  furrows  of  which,  e^'en  in  man,  the  taste-buds  are  plentiful. 
Additional  situations,  in  which,  however,  the  taste-buds  are  very  sparingly 
and  uncertainly  distributed,  include  the  fungiform  papillae,  the  soft  palate, 
the  posterior  surface  of  the  epiglottis  and  the  mesial  surface  of  the  arytenoid 
cartilages.  Within  the  fungiform  papillae  a  few  buds  may  be  found  on  the 
free  surface,  where  the  epithelium  is  thinnest.  Over  the  soft  palate  their  dis- 
tribution is  irregular  and  uncertain,  while  in  the  larynx  the  buds  are  limited 
to  the  areas  covered  by  squamous  epithelium. 

Wherever  found,  the  taste-buds  consist  exclusively  of  epithelial  tissue 
and,  in  correspondence  with  other  sense-organs,  include  two  chief  ^'arieties 
of  elements — the  supposing  cells  and  the  more  highly  specialized  neuro- 
epithelium,  the  giistatoiy  cells,  among  which  lie  the  terminal  fibrillae  of  the 
nerve  of  taste. 

The  supporting  cells  are  represented  principally  by  elongated  epithe- 
lial elements  that  occupy  both  the  superficial  and  deeper  parts  of  the  taste- 
buds  of  which  they  contribute  the  chief  bulk.  They  vary  in  their  individual 
contour,  being  lanceolate,  wedge-shaped  or  columnar,  according  to  the 
modelling  to  which  they  are  subjected  by  the  neighboring  cells.  They  pos- 
sess large  clear  vesicular  nuclei  that  contain  little  chromatin  and,  therefore, 
stain  faintly.  The  position  of  the  nucleus  is  inconstant,  in  some  cells  being 
near  the  base  and  in  others  in  the  middle  or  nearer  the  apex.      The  periph- 


THE   TASTE-BUDS. 


387 


Vc'w^' 


Taste-bud 


Taste-pore 


eral  ends  of  the  supporting  cells,  somewhat  blunted  and  flattened  and  be- 
set with  a  narrow  cuticular  zone,  are  closely  grouped  to  bound  the  annular 

opening  of  the  inner  taste-pore,  through  which  project  the  stif?  hair- processes 

of   the    gustatory  cells.      Their    deeper  or 

central  ends  are  prolonged  into  one  or  more 

protoplasmic    processes    which    unite   with 

similar    extensions    of   the    basal    cells,    as 

the  peculiar  supporting  cells  at  the  base  of 

the  bud   are  called.      The    dasa/  cells  are 

modified  sustentacular  elements,   probably 

epithelial  in  nature,  which  occupy  the  lower 

fourth  of  the  buds,  resting  upon  the  sub- 
jacent   epithelium    and,   in   turn,  affording 

support  for  the  elongated  cells.      Although 

differing    in    size  and  details  of  form,   the 

basal  cells  are  provided  with    oval    nuclei 

and  are  generally  more  or  less  branched. 
The  gustatory  cells  are  irregularly 

arranged  between  the  more  deeply  placed 

supporting  cells  and  enclosed  wuthin  a  shell 

formed  by  the  more  superficial  ones.     They 

are  long  and   fusiform,   reaching  from  the 

base    of   the  bud  to  the  inner    taste-pore, 

through  which  the  stiff"  hair-like  processes 

that  cap  their  outer  ends  project.      Their 

slender  nuclei,  rich  in  chromatin  and  deeply 

staining,   occupy  the  thickest  parts  of  the  cells,  which  beyond  the  nucleus 

are  continued  in  either  direction  as  thin  processes.      The  peripheral  ones,  as 

noted,  extend  not  only  as  far  as  the  inner  taste-pore,  but  through  the  latter 

and  into  the  canal  by  means  of  the 

--^  ■^^^:2:^  gustatory  hairs  into  which  the  taste 

'  ¥e^^^  cells  are   prolonged.      The  centrally 

directed  ends  are  usually  much  the 


Epithelium |W^  (^"^ 


Taste-bud i-J-\.  ^  ^S^^>- 

Fig.  436. — Taste-buds  in  section  ;  upper 
one  shows  gustatory  hairs  projecting  into 
taste-pore.    X  440. 


Fig.  437. — Diagram  illustrating  archi- 
tecture of  taste-bud;  i,  pore-canal;  2,  3, 
outer  and  inner  taste-pores  ;  4,  gustatory 
cell ;  5,  supporting  cell ;  6,  lymph-spaces  ; 
7,  basal  cell;  8,  sheath-cell.    \Graberg.) 


Fig.  438. — Partially  separated 
cells  of  taste-bud  with  terminal 
filaments  of  gustatorj'  nerve. 
X  510.     (Arnsiein.) 


shorter  and  join  the  processes  of  the  basal  cells.  The  number  of  gustatory  cells 
within  a  single  taste-bud  varies,  in  exceptional  cases  only  two  or  three  being 
present,  but  more  often  they  are  almost  as  numerous  as  the  supporting  cells. 
The  nerves  distributed'  to  the  gustatory  bodies  are  the  fibres  of  the 
glosso-pharyngeal,  the  nerve  of  taste.  From  the  rich  subepithelial  plexus 
numerous  twigs  ascend  into  the  epithelium,  one  set  going  directly  into  the 
taste-buds  and  the  other  ending  within  the  surrounding  tracts  of  epithelium. 


388  NORMAL   HISTOLOGY. 

The  last  set — the  interbulbai'  fibres — probably  have  no  concern  with  the  im- 
pressions of  taste  and  serve  to  convey  sensory  stimuli  of  other  value.  After 
I'epeated  division,  their  ultimate  fibrillEe  terminate  in  minute  bead-like  endings 
that  lie  free  between  the  epithelial  cells.  The  nerves  distributed  to  the  taste- 
buds — the  intrabidbar  fibres— &rv\.QX  at  the  basal  pole.  Usually  from  two  to 
five  for  each  bud,  on  gaining  the  interior  of  the  latter  they  undergo  rapid 
division.  A  majority  of  the  resulting  fibrillae  ascend  in  tortuous  windings 
towards  the  apex  of  the  bud  in  the  vicinity  of  which  some  end,  while  others 
recurve  and  end  at  lower  levels.  The  fibrillae  terminate  in  free,  usually 
minute  knob-like  endings,  that  lie  between  and  in  close  contact  with  the  sup- 
porting and  gustatory  cells.  It  is  probable  that  in  no  instance  do  the  nerve- 
fibrillae  actually  unite  with  the  gustatory  cells,  the  relation  being  one  of  appo- 
sition and  not  of  continuity. 


APPENDIX: 

INCLUDING   THE   MOST   USEFUL    METHODS   OF    HISTOLOG- 
ICAL  TECHNIQUE. 

With  the  exception  of  the  fluid  tissues,  as  blood  and  lymph,  scrapings 
from  organs,  as  the  spleen  and  the  liver,  or  ' '  teased ' '  fragments  separated 
with  needles,  as  connective  tissue  or  nerve-fibres,  the  tissues  and  organs  of 
the  body  are  so  compact  and  opaque  that  very  thin  sections,  made  transpar- 
ent by  artificial  means,  are  necessary  for  satisfactory  microscopical  examina- 
tion. Moreover,  in  order  to  display  the  structural  units,  it  is  usually  desir- 
able to  stain  such  sections  with  suitable  dyes,  so  that  advantage  may  be 
taken  of  the  differences  in  the  color  affinity  of  the  various  parts  of  the  cells 
or  of  the  intercellular  substances  to  secure  adequate  differentiation.  After 
being  stained,  the  sections  are  rendered  transparent  and  enclosed  in  some 
mounting  medium,  after  which  they  may  be  preserved  often  for  years  with- 
out deterioration. 

The  methods  devised  or  modified  by  the  many  engaged  in  histological 
work  have  resulted  in  the  great  mass  of  technical  details  described  in  the 
various  books  devoted  to  the  subject.  Notwithstanding  the  \'alue  of  special 
processes  for  particular  lines  of  investigation,  by  far  the  greater  part  of  his- 
tological work  is  accomplished  with  a  few  well-tried  standard  methods.  To 
describe  the  most  useful  of  these  methods,  as  carried  out  in  the  laboratory, 
for  the  assistance  of  the  student  who  may  wish  to  undertake  the  preparation 
of  material  for  microscopical  examination,  these  pages  are  added.  No  at- 
tempt is  made  even  to  mention,  much  less  describe,  many  excellent  methods 
for  particular  purposes.  The  few  procedures  here  given,  however,  may  be 
depended  upon  to  yield  excellent  results,  when  properly  carried  out,  and  in 
the  great  majority  of  cases  will  be  found  to  be  adequate  for  the  demonstra- 
tion of  structural  details.  The  student  undertaking  independently  such  work 
for  the  first  time  is  urged  to  persevere  with  the  methods  here  given  until  he 
has  repeatedly  carried  them  to  the  successful  results  of  which  they  are  ca- 
pable. Failures,  sure  to  beset  the  beginner,  should  be  carefully  analyzed  and 
be  made  to  yield  experience  guarding  against  their  repetition. 


FIXATION   AND    PRESERVATION   OF   TISSUES. 

It  is  evident,  that,  no  matter  how  carefully  subsequent  manipulations 
be  conducted,  unless  the  tissue  itself  be  successfully  preserved,  the  finished 
preparation  will  not  present  a  trustworthy  picture  of  the  normal  structure. 
The  tissue  must  be  secured,  therefore,  as  fresh  as  possible,  since  in  the  case 
of  deUcate  structures,  as  the  epithelium  lining  the  digesti\'e  canal,  the 
post-mortem  changes  occurring  within  a  few  hours  are  sufficient  to  destroy 
interesting  details.  Tissues  from  the  lower  animals  are  readily  obtained  from 
the  recently  killed  animal,  while  still  warm  and  the  cells  yet  alive;  those  from 
man  are  less  easily  secured,  the  early  autopsy  and  some  favorable  operation 
being  the  usual  sources  for  the  histologist' s  stock.  The  manifest  desidera- 
tum being  to  retain,  as  far  as  possible,  structural  details  in  the  condition  in 

389 


390  APPENDIX. 

which  they  existed  during  life,  it  is  necessary  to  kill  the  tissue  rapidly,  other- 
wise all  evidence  of  certain  phases  of  cellular  activity,  as  the  figures  of  mitotic 
division,  may  disappear  during  the  slow  death  of  the  cells. 

This  rapid  killing  of  the  tissues,  known  as  fixation,  is  accomplished  by 
plunging  the  fresh,  preferably  still  warm,  material  into  some  suitable  fixing 
solution.  The  solutions  for  this  purpose  are  many,  since  some  tissues  fix 
well  in  certain  fluids,  while  in  the  same  ones  other  tissues  may  be  only  in- 
differently preserved.  The  general  precautions  to  be  observed  in  fixing 
include:  material  should  be  in  as  small  pieces  as  practicable,  never  over  2  cm. 
thick  and  better  not  more  than  half  as  much.  Otherwise  the  penetration  is 
incomplete  with  corresponding  imperfect  fixation.  The  volume  of  fixing 
fluid  should  be  many  (fifty  or  more)  times  that  of  the  tissue.  Further,  the 
fluid  should  be  changed  whenever  it  becomes  turbid.  This  often  happens 
within  the  first  few  hours  after  the  introduction  of  the  tissue.  The  latter 
should  not  be  washed  in  water  on  being  removed  from  the  animal,  but  placed 
directly,  with  the  minimum  handling,  in  the  fixing  fluid.  A  cushion  of  ab- 
sorbent cotton  affords  desirable  support  and  insures  access  of  the  reagent  on 
all  sides.     Among  the  most  useful  fixing  reagents  are  the  following. 

Zenker's  Fluid. 

Potassium  bichromate 2.5  gm. 

Sodium  sulphate i  gm. 

Mercury  bichloride 5  gm. 

Distilled  water,  warm 100    cc. 

Just  before  using,  to  each  20  cc.  of  the  above  solution  add  i  cc.  of 
glacial  acetic  acid. 

Place  small  pieces  of  tissue  in  a  generous  amount  of  the  fluid  for  10-24 
hours  ;  then  wash  in  running  water  for  1 2-24  hours  ;  transfer  for  a  few  hours 
to  alcohol  of  increasing  (50,  65,  80)  strength,  in  the  strongest  of  which  keep 
until  used.  This  fluid  is  an  excellent  fixative  for  cell-structure,  but,  in  com- 
mon with  other  solutions  containing  sublimate,  has  the  disadvantage  of  re- 
quiring subsequent  special  treatment  to  remove  the  crystals  of  the  mercuric 
chloride  that  are  commonly  deposited  throughout  the  tissue.  The  removal 
of  the  deposits  of  mercury  is  conveniently  effected  by  placing  the  unstained 
wet-cut  sections  for  15  minutes  in  iodine  alcohol  (3  drops  tincture  of  iodine 
to  15  cc.  90  per  cent,  alcohol).  An  additional  15  minutes  in  a  dilute  solution 
of  sodium  hyposulphite  (made  by  adding  10  cc.  of  a  2.5  per  cent,  aqueous 
solution  of  the  salt  to  100  cc.  distilled  water)  insures,  in  turn,  the  removal  of 
the  iodine.  If,  however,  the  tissue  is  to  be  stained  in  bulk  and  cut  in  par- 
affin, the  deposits  of  mercury  must  be  eliminated  by  adding  the  iodine  (from 
.25  to  .5  per  cent,  of  Lugol's  solution)  to  the  alcohol  in  which  the  tissue  is 
kept  after  fixation.  The  manipulator  must  assure  himself,  preferably  by  an 
examination  of  a  free-hand  section,  of  the  disappearance  of  the  deposits, 
otherwise,  their  removal  after  the  tissue  has  been  embedded  in  paraffin  is 
troublesome  and  often  impracticable, 

Tellyesnizcky's  Fluid. 

Potassium  bichromate 3  gm. 

Distilled  water 100    cc. 

To  which  is  added  just  before  using 

Glacial  acetic  acid 5    cc. 


APPENDIX.  391 

Moderate  sized  pieces  of  tissue  remain  in  the  solution  18-24  hours;  then 
are  washed  in  running  water  3-5  hours  ;  and  carried  through  50,  70  and  95 
alcohol,  each  for  one  day.      This  fluid  is  excellent  for  large  embryos. 

Miiller's  Fluid. 

Potassium  bichromate 25-30  gm. 

Sodium  sulphate 10  gm. 

Water 1000   cc. 

This  classic  fixing  fluid  requires  prolonged  action,  from  two  to  eight 
weeks  or  longer,  and  a  large  excess  in  proportion  to  the  volume  of  the  ma- 
terial. During  the  first  week,  the  fluid  should  be  changed  daily,  or  in  the  be- 
ginning oftener  if  it  becomes  turbid  ;  thereafter,  each  week.  After  a  variable 
time,  the  tissue  is  washed  in  running  water  for  12—24  hours  and  then  trans- 
ferred to  70  alcohol  and  placed  in  the  dark,  with  occasional  renewal  of  the 
spirit.  If,  however,  the  material  be  nervous  tissue  intended  for  subsequent 
treatment  with  special  methods,  such  as  Weigert  staining,  it  is  transferred, 
after  slight  rinsing,  directly  to  the  alcohol.  This  classic  fluid,  although 
hardening  e\-enh'  and  without  shrinkage  large  masses,  as  entire  small  brains 
and  cords,  has  the  disadvantage  of  being  not  only  tedious  but  also  lacking 
in  accurate  nuclear  fixation.  Since,  however,  it  is  the  basis  of  several  com- 
binations of  value,  its  preparation  as  a  stock  solution  is  desirable. 

Orth's  Fluid. 

Miiller's  fluid 100  cc. 

Formalin  1^40  p. c.  solution) 10  cc. 

The  two  solutions  are  mixed  just  before  using  and  the  tissue,  cut  into 
pieces  i  cm.  or  less  in  thickness,  allowed  to  remain  not  over  four  days,  with 
two  changes.  Thorough  washing  in  running  water  for  18-24  hours  is  im- 
portant, followed  by  80  alcohol.  This  fluid  yields  excellent  results,  the  cel- 
lular fixation  being  satisfactory  and  the  required  time  not  excessive.  Small 
pieces  of  tissue,  not  exceeding  5  cm.  in  thickness,  may  be  fixed  and  hard- 
ened within  a  few  hours  if  kept  at  a  temperature  of  45°  C.  in  an  oven. 

Formalin  Solution. 

Formalin  (40  p. c.  solution) 10  cc. 

Water 90  cc. 

Objects  remain  for  48  hours,  or  longer,  and  are  then  transferred  to  95 
alcohol  for  at  least  two  days.  This  4  per  cent,  aqueous  solution  of  formal- 
dehyde has  con\-enience  as  its  chief  recommendation,  since,  when  used  alone, 
it  is  deficient  as  an  accurate  fixative  and  often  does  not  favor  staining. 
Where,  however,  the  gross  features  of  a  specimen  are  to  be  supplemented 
by  microscopic  examination,  formalin  offers  a  satisfactory  compromise  for  the 
anatomist  or  pathologist  and  the  histologist.  Another  important  use  of  this 
formalin  solution  should  be  remembered,  namely,  the  preservation  of  em- 
bryos. The  reagent  is  so  readily  obtained,  that  the  obstetrician,  upon  whom 
the  embryologist  must  depend  for  his  supply  of  human  material,  can  secure, 
with  little  trouble,  valuable  specimens  which  are  too  often  lost. 

Flemming's  Solution. 

Osmic  acid  (2  p.c.  aqueous  solution)     ......      4  parts. 

Chromic  acid  ( I  p. c.  aqueous  solution) 15    " 

Glacial  acetic  acid i    " 


392  APPENDIX. 

The  mixture  is  best  prepared  just  before  using,  although  it  will  keep  for 
some  time  without  serious  deterioration,  if  tightly  stoppered.  Owing  to  the 
cost  of  the  osmic  acid,  the  solution  should  be  used  with  economy.  The 
tissue,  in  pieces  not  over  a  few  millimeters  in  size,  remains  in  the  mixt- 
ure for  1-2  days,  or  even  longer  ;  is  then  washed  in  running  water  for  12-24. 
hours  and  transferred,  by  ascending  strengths,  to  80  alcohol,  in  which  it  may 
be  preser\'ed,  the  spirit  being  changed  when  cloudy.  This  reagent,  al- 
though somewhat  expensive,  is  of  great  value  in  studies  concerning  cell- 
structure  and  mitosis,  being  one  of  the  most  reliable  and  accurate  means  of 
fixation.  Its  power  of  uniform  penetration,  however,  is  very  limited  ;  it  is 
necessary,  therefore,  to  use  pieces  as  small  as  possible,  otherwise  the  desir- 
able action  of  the  osmic  acid  will  be  confined  to  the  peripheral  zone,  whilst  the 
deeper  parts  of  the  tissue  will  be  influenced  chiefly  by  the  chromic  acid  alone. 

Absolute  Alcohol. — This  reagent  is  useful  for  fixing  and  hardening 
certain  tissues,  as  glands  and  blood-vessels,  which  remain  in  it  at  least  24 
hours,  although  the  time  may  be  extended  to  several  days.  The  alcohol 
should  always  be  changed  within  3-4  hours  and  the  tissue  supported  by  a 
layer  of  absorbent  cotton.  It  is  important  that  the  alcohol  be  approximately 
"absolute,"  since,  even  when  of  96  per  cent,  its  action  is  very  different. 

The  hardening  of  the  tissues  preparatory  to  cutting  sections  in  the 
usual  way,  that  is,  after  interstitial  embedding,  is  of  much  less  consequence 
than  accurate  fixation,  since,  unless  the  material  be  unduly  hardened,  the 
consistence  of  the  mass  sectioned  is  largely  determined  by  that  of  the  sup- 
porting material — the  celloidin  or  paraffin. 

EMBEDDING   AND   CUTTING   SECTIONS. 

Having  secured  properly  fixed  and  preserved  tissues  by  one  or  more  of 
the  preceding  methods,  such  tissue  must  be  cut  into  thin  sections,  stained, 
rendered  transparent  and  mounted  before  it  can  be  satisfactorily  examined 
under  the  microscope.  Whether  sectioning  or  staining  takes  precedence 
depends  upon  the  character  of  the  object  and  the  end  in  view.  If  the  object 
be  an  embryo  or  some  structure  for  whose  study  it  is  desirable  to  secure  a 
series  of  sections'in  strict  sequence,  or  sections  of  the  least  possible  thickness, 
the  pa^'offin  method  is  chosen.  If,  however,  these  particular  features  are  un- 
important and  the  production  of  thoroughly  good  preparations  for  general 
histological  study  is  the  result  in  mind,  the  celloidin  method  offers  advantages 
and  is  usually  selected.  Further,  if  it  is  desired  to  cut  serial  sections  of  tissue 
requiring  only  a  single  staple  stain,  as  carmine  or  hematoxylin,  to  demon- 
strate satisfactorily  its  general  structure,  much  labor  is  saved  and  risk  of  los- 
ing sections  avoided  by  staining  the  object  in  toto  before  sectioning  in  par- 
affin. Unless  this  is  done,  the  supporting  paraffin  must  be  removed  by 
a  solvent  (xylol)  and  the  sections,  previously  fixed  to  the  slide,  stained  in 
position,  a  procedure  requiring  considerable  time  and  care  when  long  series 
are  to  be  treated.  Since,  except  for  embryos  and  special  work,  the  reten- 
tion of  the  sections  in  strict  sequence  is  not  usually  necessary,  staining  in 
bulk  is  much  less  frequently  followed  than  staining  after  cutting.  When  cut 
in  celloidin  the  sections  may  be  conveniently  treated  in  large  numbers  at  one 
time  and  contrast  dyes  employed  with  little  additional  trouble. 

Whichever  method  be  chosen,  celloidin  or  paraffin,  the  fundamental 
principle  is  the  same — the  complete  impregnation  or  saturation  of  the  object 
with  the  embedding  mass,  so  that  when  the  tissue  is  cut  into  thin  sections 
even  the  most  delicate  and  isolated  parts  shall  be  retained  in  position  by  the 


APPENDIX.  393 

support  of  tl  e  mass.  This  constitutes  interstitial  embedding,  as  distinguished 
from  superjisial,  by  which  the  object  is  merely  surrounded  with  some  ma- 
terial, as  tal  ow  and  wax,  that  affords  superficial  support  but  does  not  bind 
together  the  structural  components.  In  either  case,  whether  celloidin  or  par- 
affin, the  tissue  must  be  carefully  dehydrated  before  being  placed  in  the  fluid 
embedding  naterial.  Upon  the  proper  carrying  out  of  this  step  depends  the 
success  or  fa  lure  of  the  embedding,  since  if  water  still  remains  within  the  ob- 
ject, the  em  )edding  substance  fails  to  penetrate  its  deeper  parts  and  the 
interstitial  embedding  is  incomplete. 

Dehydration  must  be  thorough,  for  unless  all  the  water  be  extracted 
from  the  object  the  subsequent  penetration  of  the  embedding  substance  will 
be  imperfect  and  the  sections  uneven  or  torn.  This  essential  step  is  effected 
by  alcohol.  Assuming  that  the  tissue  has  been  stored  in  70  or  80  spirit,  the 
usual  strength  for  this  purpose,  it  is  transferred  to  a  tightly-stoppered  wide- 
mouth  bottle,  of  200  cc.  capacity,  containing  95  alcohol,  for  24  hours.  It 
is  then  placed  in  a  similar  bottle,  of  100  cc.  capacity,  containing  absolute 
alcohol,  for  24-48  hours,  during  which  the  absolute  alcohol  should  be 
changed.  The  exact  length  of  time  needed  for  complete  dehydration  varies 
with  the  density  and  the  size  of  the  object,  delicate  small  objects,  such  as 
small  embryos,  evidently  requiring  much  less  time  than  compact  and  large 
ones.  It  is  always  better  to  allow  ample  time  for  the  extraction  of  the  water, 
since  undue  haste  in  this  particular  may  require  tedious  retracing  of  subse- 
quent steps. 

Celloidin  Embedding, — As  now  supplied  by  Schering,  celloidin  is 
purchased  in  small  hard  pieces,  or  "  shreds,"  which  are  packed,  surrounded 
by  water,  in  ounce  lots  in  wide-mouth  bottles.  Although  immersed  in  water, 
the  celloidin  does  not  absorb  the  fluid  and,  after  drying  its  surface,  is  really 
free  from  moisture  and  desiccated.  In  order  to  remove  the  adherent  water, 
the  shreds  are  allowed  to  dry  in  the  air,  protected  from  dust,  or  they  are 
first  washed  with  alcohol  to  hasten  the  process.  When  perfectly  dry,  the 
shreds  of  celloidin  are  broken  into  small  pieces  and  dissolved  and  solutions 
prepared  as  follows: 

Solution  A. — Dried  celloidin  16  grams,  dissolved  in  mixture  of  absolute 
alcohol  and  ether,  100  cc.  each.  When  completely  dissolved,  which  re- 
quires some  days  with  occasional  stirring,  the  solution  has  the  consistence  of 
thick  syrup;  it  is  necessary,  therefore,  to  prepare  a  less  concentrated  one. 
Solution  B  is  made  by  taking  half  of  solution  .\  and  diluting  with  50  cc. 
each  of  absolute  alcohol  and  ether.  Solution  C,  made  in  a  similar  way  by 
taking  half  of  B  and  diluting  with  25  cc.  each  of  absolute  alcohol  and  ether, 
is  useful  for  dense  tissue,  as  it  insures  thorough  penetration.  It  is,  however, 
often  omitted,  the  object  being  placed  at  once  into  B. 

The  tissue  having  been  thoroughly  dehydrated,  the  actual  embedding  is 
accomplished  by  the  following  manipulations: 

I.  Dehydrated  tissue  placed  in  mixture  of  equal  parts  of  absolute  alco- 
hol and  ether  for  24  hours. 

7.   Transfer  to  thin  (C)  celloidin  solution  for  24  hours. 

3.  Transfer  to  medium  (B)  celloidin  solution  for  24-48  hours  or  longer. 

4.  Transfer  to  thick  (A)  celloidin  solution  for  at  least  24  hours  ;  may 
remain  in  this  for  much  longer  time,  often  with  advantage  for  a  week  if  the 
object  be  large. 

.  5.  Mount  on  small  blocks  of  \'ulcanized  fibre,  one  surface  of  which  is 
first  coated  with  a  thin  layer  of  thick  celloidin  solution.  After  arranging  the 
object  in  position,  making  sure  that  it  rests  firmly  on  the  block  by  a  broad 


394  APPENDIX. 

surface,  it  is  necessary  to  pour  over  it  additional  thick  celloiiin  to  insure 
its  complete  enclosure.  Expose  to  the  air  for  a  few  minutes,  until  the  cel- 
loidin  has  set  and  the  object  is  securely  attached. 

6.  Place  block  with  object  in  80  alcohol  to  harden  celloidm,  which  will 
require  12-24  hours,  until  sections  are  cut.  Blocks  are  stored  n  80  alcohol, 
where  they  may  remain  for  years.  Should  the  embedded  object  separate 
from  the  block,  as  sometimes  happens  during  cutting,  the  j  arface  of  the 
block  must  be  cleaned,  a  new  layer  of  thick  celloidin  applied,  ;  nd  the  hard- 
ening in  80  alcohol  be  repeated.  In  general,  the  smaller  the  object  and  the 
harder  the  celloidin  the  thinner  can  sections  be  cut.  The  advantages  of  the 
celloidin  method  include  :  ( i )  avoidance  of  heating  the  tissvies,  which  if  in- 
judiciously done  may  distort  or  altogether  ruin  them  ;  (2)  simplicity  of  pro- 
cedure, neither  embedding  oven  and  accessories  nor  close  attention  being 
required  ;  and  (3)  ready  application  of  various  methods  of  staining.  The 
chief  disadvantages  of  the  method  are :  ( i )  difficulty  of  obtaining  extremely 
thin  sections;  (2)  impracticability  of  preserving  the  sequence  of  sections, 
unless  special  means  are  employed  ;  (3)  length  of  time  ordinarily  required 
to  prepare  object  for  sectioning.  All  in  all,  the  celloidin  method  is  to  be 
preferred  for  routine  histological  work  of  the  beginner,  although  for  many 
special  purposes  and  for  embryological  material  it  is  much  less  satisfactory 
than  paraffin. 

Paraffin  Embedding. — Although  invaluable  for  work  demanding 
serial  or  very  thin  sections,  the  paraffin  method  requires  some  means  of 
maintaining  a  constant  temperature.  With  care  and  attention  this  may  be 
done  with  a  simple  water  bath,  but  for  serious  work  a  suitable  embedding 
oven  is  almost  a  necessity.  This  should  be  made  of  copper,  have  double 
walls  enclosing  a  water-space,  and  be  supplied  with  a  suitable  door  on  one 
of  the  larger  sides  of  the  rectangular  box.  The  water-space  must  be  pro- 
vided with  at  least  one  opening  large  enough  to  admit  the  bulb  of  a  gas- 
regulator  and  the  supporting  perforated  cork.  A  second  aperture,  with 
tubular  collar,  should  lead  through  the  water-space  from^  the  top  into  the 
interior  of  the  warm  chamber  and  serve  for  the  passage  and  support  of  the 
thermometer.  The  bulb  of  the  latter  should  occupy  the  approximate  centre 
of  the  oven.  One  or  two  perforated  copper  shelves,  not  less  than  6  cm. 
apart,  provide  space  for  half  a  dozen  or  more  embedding  capsules.  Since 
the  temperature  to  be  maintained  is  from  50-55°  C. ,  some  form  of  "micro" 
burner  is  necessary.  If  possible,  a  "  blue  flame"  burner  should  be  used  to 
avoid  the  gradual  accumulation  of  soot  deposited  from  the  ordinary  gas  jet. 
Even  the  miniature  Bunsen  burners  often  give  too  much  heat  unless  the  bot- 
tom of  the  oven  be  raised  sufficiently  high.  Adequate  protection  of  the 
flame  from  draughts  is  important.  An  efficient  regulator  to  control  the  flow 
of  gas  is,  of  course,  necessary.  The  usual  inexpensive  form  (Reichert's)  will 
answer,  a  daily  variation  of  one  or  two  degrees  ordinarily  doing  no  harm. 
Not  infrequently  the  by-pass  is  too  large  and  the  opening  must  be  reduced 
before  a  satisfactory  temperature  is  maintained.  On  installing  a  new  oven, 
usually  some  days  are  required  in  testing  and  adjusting  it  before  valuable 
tissue  should  be  entrusted  to  prolonged  embedding. 

Paraffin  should  be  of  two  kinds,  one — the  soft — with  a  melting  point  of 
45°  C. ,  and  the  other — the  hard — melting  at  54°  C.  It  is  important  that 
the  paraffin  be  of  suitable  quality,  that  supplied  by  Griibler  being  recom- 
mended. The  quantity  used  is  so  inconsiderable,  that  the  slightly  increased 
cost  is  of  little  consequence  in  comparison  with  the  satisfaction  of  having  a 
dependable  article.     Neither  of  the  above  grades  is  used  alone,  but  a  mixt- 


APPENDIX. 


395 


ure  of  the  two,  so  proportioned  that  the  resulting  mass  has  a  meljing  point 
of  about  52°  C.  The  proportions  vary  so  with  the  season  and  temperature 
of  the  laboratory  that  no  exact  figures  can  be  given,  but,  in  a  general  way, 
about  equal  parts  will  yield  a  satisfactory  mass  for  ordinary  use.  When  the 
object  is  relatively  hard  and  very  thin  sections  are  desired,  harder  paraffin  is 
employed  than  usual.  A  nice  adjustment  of  the  consistence  of  the  embed- 
ding mass  to  the  character  of  the  tissue  and  the  particular  purpose  in  view  is 
an  important  factor  in  securing  satisfactory  results. 

The  manipulations  in  paraffin  embedding  are  as  follows  : 

1.  The  thoroughly  dehydrated  tissue  is  transferred  from  absolute 
alcohol  to: 

2.  Chloroform,  4-24  hours,  depending  upon  the  size  of  the  object, 
small  delicate  embryos  requiring  often  only  1-2  hours.  When  the  alcohol  is 
completely  replaced,  the  tissue  floats  below  the  surface  or  sinks. 

3.  Saturated  solution  of  paraffin,  in  chloroform,  6-24  hours,  depend- 
ing upon  the  size  and  density  of  the  object. 

4.  Melted  paraffin  in  small  open  porcelain  or  glass  round-bottom 
dish,  2-12  hours,  in  oven  at  52°-54°  C.  The  paraffin  must  be  carefully 
guarded  against  overheating  to  prevent  injury  to  the  tissue.  After  1-2 
hours,  the  object  should  be  transferred  to  a  second  capsule  with  fresh  melted 
paraffin.  So  long  as  chloroform  is  present,  the  embedding  is  incomplete. 
In  order  to  test  these  conditions,  a  rod  may  be  judiciously  heated  and  held 
for  a  few  moments  in  the  paraffin  in  the  vicinity  of  the  object,  when,  if  still 
present,  the  chloroform  is  liberated  as  small  bubbles.  When  the  chloroform 
has  been  driven  off,  the  object  is  transferred  once  more  to  fresh  paraffin  pre- 
paratory to  embedding.  This  last  transference  is  delayed  until  there  is  reason 
to  believe  that  the  object  is  completely  saturated  with  the  embedding  mass 
and  free  from  chloroform,  since  the  presence  of  the  latter  is  unfavorable  to 
the  desired  homogeneity  of  the  embedding  mass. 

5.  Embedding  is  accomplished  by  surrounding  the  object  with  melted 
paraffin  in  a  folded  paper  box  or,  still  better,  an  adjustable  metal  frame. 
When  an  embedding  frame  is  employed — and  its  convenience  recommends 
it — it  is  adjusted  to  suitable  size  and  placed  upon  a  piece  of  glass  resting  on 
the  flat  bottom  of  a  deep  dish.  The  frame  and  sheet  of  glass  should  be 
warmed  to  prevent  the  too  rapid  solidifying  of  the  paraffin.  The  object 
may  be  transferred  to  the  embedding  box  by  pouring  when  the  box  is 
filled,  or  by  transferring  with  a  warmed  loop  of  platinum  wire.  Before 
the  paraffin  ceases  to  be  fluid,  the  position  of  the  object,  with  regard  to 
the  desired  planes  of  section,  must  be  carefully  adjusted  by  means  of  heated 
needles. 

When  the  proper  orientation  of  the  object  has  been  secured,  cold 
water  is  poured  into  the  dish,  care  being  taken  neither  to  shake  the  object, 
and  thereby  disturb  its  position,  nor  to  allow  the  water  to  rise  above  the 
sides  of  the  frame.  The  mass  is  allowed  to  remain  for  a  few  moments, 
until  the  surface  of  the  paraffin  is  completely  covered  by  a  thin  pellicle  of 
congealed  substance.  When  this  has  occurred,  the  water  is  very  gently 
added  and  the  entire  frame  and  contents  submerged.  If  the  water  be 
allowed  to  come  into  contact  with  the  still  fluid  paraffin,  cavities  containing 
water  may  be  imprisoned  within  the  block — an  accident  that  may  interfere 
with  proper  cutting.  In  order  to  secure  homogeneous  paraffin,  a  most 
desirable  feature  for  satisfactory  sectioning,  it  is  necessary  to  solidify  the 
melted  mass  as  rapidly  as  may  be  done  with  prudence.  If  it  be  allowed  to 
cool  slowly,  crystallization  occurs  and  the  mass  becomes  opaque  and  friable. 


396  APPENDIX. 

After  thorough  cooHng  and  hardening,  the  frame  is  removed  and  the  mass, 
if  still  adherent  to  the  glass  plate,  carefully  loosened.  Tissue  properly  em- 
bedded in  parafhn  may  be  kept  for  years  without  deterioration,  if  guarded 
against  dust  and  excessive  temperatures.  Indeed,  delicate  objects,  as  em- 
bryos, are  much  more  satisfactorily  preserved  after  being  carefully  embedded 
than  if  kept  for  a  long  period  in  alcohol.  It  is  often  convenient  and  eco- 
nomical to  embed  t\\o  or  even  more  small  objects  at  one  time  in  the  same 
block,  care  being  taken  to  mark,  by  some  ineffaceable  label,  the  character 
and  cutting  planes  of  the  enclosures. 

Several  methods  of  double  embedding  have  been  devised  with  a  view 
of  combining  the  advantages  of  celloidin  with  those  of  paraffin.  For  particular 
lines  of  work  they  are  valuable  and  fairly  satisfactory.  Their  description 
will  be  found  in  the  special  books  on  microscopical  technique. 

Cutting  Sections. —  Although  thoroughly  adequate  sections  may 
often  be  obtained  "  free-hand  "  with  a  razor  or  suitable  knife,  for  the  most  sat- 
isfactory results  an  accurately  constructed  microtome  is  a  necessity.  If  pos- 
sible, a  medium-sized  instrument — neither  too  small  and  light  nor  too  large 
and  cumbersome — should  be  chosen.  As  an  all-round  microtome,  the  type 
made  by  Schanze  leaves  little  to  be  desired,  as  it  may  be  used  with  satisfac- 
tion for  all  kinds  of  cutting,  wet  or  dry,  and  is  accurate  and  simple.  If  only 
one  knife  is  available,  one  of  the  Weigert  form,  with  a  straight  slotted  shank 
at  an  angle,  is  to  be  preferred.  If  much  ' '  ribbon-cutting  "  is  to  be  done,  a 
special  holder  should  be  provided,  by  which  the  knife  can  be  clamped  at  dif- 
ferent points  and  successive  portions  of  the  blade  be  brought  into  use.  For 
serial  sections,  the  Minot  rotary  microtome  is  most  convenient. 

Celloidin  sections  are  cut  wet,  both  object  and  knife  being  kept  con- 
tinuously moistened  with  80  alcohol.  Preparatory  to  using  the  microtome, 
it  is  advisable  to  remove  the  superfluous  embedding  material  with  a  sharp 
razor  or  knife,  leaving  a  narrow  zone,  1-2  mm.  in  width,  surrounding  the 
object.  One  or  more  sections  across  the  trimmed  block  are  then  made  to 
expose  the  surface  to  be  sectioned.  The  fibre-block  bearing  the  embedded 
tissue  is  now  securely  clamped  in  the  object  holder  and  adjusted  as  to  plane 
of  section  and  height.  It  is  important  not  to  raise  the  object  too  much  since, 
if  the  removal  of  a  slice  of  too  great  thickness  be  attempted,  the  entire  mass 
of  celloidin  may  be  torn  from  the  fibre-block.  The  desired  level  of  the  tissue 
should  be  gained,  therefore,  by  repeated  thin  sections,  thereby  giving  the 
opportunity  to  adjust  the  knife  and  object  in  the  mutually  most  favorable 
position.  The  knife  is  set  at  such  an  angle  that,  after  striking  the  corner  of 
the  object,  it  passes  through  the  latter  obliquely  and  completes  the  section 
before  coming  to  the  end  of  the  "  track."  The  section  is  removed  from  the 
blade  and  placed  in  a  dish  containing  80  alcohol.  The  knife  is  returned  to 
the  beginning  of  its  track,  the  object  raised  by  the  micrometer  screw  to  the 
desired  thickness  of  the  next  section,  and  the  blade  again  drawn  through  the 
block  with  a  steady  unhesitating  pull. 

Paraffin  sections  are  cut  dry.  When  the  object  is  large  and  not 
delicate,  the  paraffin  block  may  be  clamped  directly  in  the  jaws  of  the  object 
holder.  If,  however,  the  object  is  of  small  size  and  great  delicacy,  as  an 
embryo,  the  block  of  paraffin  with  the  embedded  tissue  is  fastened  to  one  of 
the  metal  "  tables  "  supplied  with  the  microtome.  This  is  best  accomplished 
by  covering  the  surface  of  the  metal  disk  with  a  thin  layer  of  melted  paraffin, 
on  which  the  embedded  object  is  firmly  pressed,  additional  security  being 
given  by  running  a  heated  knife  or  other  convenient  tool  around  the  base  of 
the  block.     After  thorough  cooling,  the  block  is  ready  for  trimming  prepar- 


APPENDIX.  397 

atory  to  sectioning.  This  should  be  done  with  a  sharp  knife,  care  being 
taken  that  in  shaping  the  block  the  shces  of  paral^n  are  not  too  thick,  lest 
the  enclosed  object  be  subjected  to  undue  pressure.  While  all  super- 
fluous embedding  mass  should  be  cut  away,  the  trimming  must  not  be  too 
close,  a  margin  of  parafifin,  one  or  two  millimeters  broad,  being  left  around 
the  object. 

For  ordinary  purposes,  the  knife,  scrupulously  clean,  is  set  at  angle  and 
carried  obliquely  through  the  tissue.  For  routine  examinations,  sections 
.007— .010  mm.  thick  will  be  sufficiently  thin,  particularly  when  of  some  size. 
If,  however,  cell-structure  and  other  details  requiring  the  use  of  high  ampli- 
fication are  in  view,  the  sections  can  not  be  too  thin.  Under  the  most  favor- 
able conditions,  it  is  possible  to  obtain  sections  of  small  size  which  are  not 
over  .001  mm.  in  thickness. 

If  the  consistence  of  the  paraffin  and  tissue  be  just  right  and  the  sec- 
tioned surface  not  too  large,  the  sections  will  lie  flat  and  smooth  on  the  knife, 
from  which  they  are  removed  as  cut  and  placed  on  a  clean  sheet  of  writing 
paper.  Protected  from  dust  and  excessive  temperature,  they  may  be  put 
aside  for  mounting  at  some  future  time.  If,  however,  they  be  allowed  to  lie 
too  long,  particularly  in  a  warm  temperature,  there  is  danger  of  their  stick- 
ing to  the  paper;  early  mounting  is,  therefore,  to  be  recommended.  After 
the  sections  desired  at  the  time  have  been  cut,  the  block  may  be  put  aside 
for  subsequent  use.  If  the  paraffin  is  hard  the  sections  will  roll  up  on 
the  knife.  This  annoying  feature  may  be  prevented  by  lightly  holding 
down  the  edge  of  the  section  with  a  small  sable  brush  in  the  left  hand  as 
the  knife  is  drawn  through  the  tissue.  In  this  manner  flat  sections  may 
often  be  secured  when,  without  the  manipulation,  they  would  be  tightly  and 
hopelessly  rolled. 

When  the  paraffin  is  too  hard,  still  contains  chloroform,  or  lacks  homo- 
geneity, it  often  is  brittle  and  crumbling,  so  that  the  sections  break  before 
completed.  Too  little  consistence  of  the  embedding  mass  is  also  unfavorable 
for  satisfactory  cutting,  since,  if  the  paraffin  be  too  soft,  insufficient  support 
is  given  the  sections,  which  then  come  off  more  or  less  wrinkled  and  com- 
pressed with  corresponding  distortion  of  the  object.  If  the  compression  is 
not  excessive,  the  sections  may  often  be  restored  to  their  normal  form  by 
floating  them  on  judiciously  warmed  water,  where  they  expand.  Obviously 
care  must  be  taken  that  the  temperature  of  the  fluid  is  not  sufficiently  high 
to  dissolve  the  supporting  paraffin.  The  temporary  reduction  of  the  tempera- 
ture of  the  room,  by  opening  an  adjacent  window,  frequently  serves  to  cor- 
rect undue  softness  of  the  embedding  mass.  Similarly,  bringing  a  gas-flame, 
or  other  source  of  heat,  into  the  vicinity  of  the  microtome  sometimes  enables 
satisfactory  sections  to  be  made  with  over-hard  paraffin.  Care  must  be 
observed  to  keep,  by  an  occasional  touch  with  a  dr)^  cloth,  the  knife  clean 
and  free  from  adherent  particles  of  paraffin,  especially  the  under  surface  of 
the  cutting  edge.  If  this  precaution  be  neglected,  an  attached  fragment  of 
paraffin  may  ruin  important  sections  by  causing  fissures  and  breaks.  A 
properly  ground  and  really  sharp  knife  is  an  indispensable  requisite  for  the 
production  of  successful  preparations. 

Serial  sections  are  most  easily  cut  in  the  form  of  "  ribbons."  While, 
of  course,  their  sequence  may  be  preserved  by  carefully  arranging  isolated 
sections  in  the  order  in  which  they  are  cut,  much  time  and  trouble  are  saved 
by  adopting  the  ribbon-method,  whereby  the  sections  are  caused  to  adhere 
and  form  a  long  chain.  The  method  is  particularly  suitable  for  small  and 
delicate  objects  stained  in  toto,  as  embryos,  which  are  to  be  entirely  cut  into 


398  APPENDIX. 

sections.  The  knife  is  set  at  right  angle  to  the  microtome  track  and  the 
paraffin  block  so  trimmed  (the  desired  plane  of  section  being  first  secured  by 
the  microtome  adjustments)  that  the  two  long  sides  of  the  rectangular  block 
are  exactly  parallel,  not  only  with  each  other  but  also  with  the  edge  of  the 
knife.  Since  the  mounted  sections  will  be  separated  by  twice  the  width  of 
the  border  of  paraffin  surrounding  the  object,  the  block  should  be  trimmed 
as  close  as  possible  to  the  tissue,  a  thin  covering  of  paraffin,  however,  being 
left  over  the  upper  surface  of  the  object. 

Everything  being  in  readiness,  the  object  is  raised  to  a  height  sufficient 
to  allow  several  sections  of  the  overlying  paraffin  being  made  without  in- 
volving the  object.  After  cutting  a  few  sections  that  include  the  entire  sur- 
face of  the  block,  the  knife  is  returned  to  the  beginning  of  the  track,  the 
block  elevated  by  the  micrometer  screw,  and,  without  removing  the  section, 
the  blade  drawn  across  the  block  for  the  next  section.  If  the  consistence  of 
the  paraffin  is  proper,  the  last  cut  section  pushes  the  preceding  one  before  it 
on  the  blade  and,  at  the  same  time,  adheres  to  it.  The  manipulation  is  re- 
peated and,  if  all  goes  well,  each  succeeding  section  adheres  to  its  prede- 
cessor and  advances  the  chain.  For  embryological  work,  in  which  ribbon- 
cutting  is  particularly  useful,  the  sections  should  be  of  uniform  thickness, 
.01  mm.  being  satisfactory  for  most  purposes.  Sections  less  than  .005  mm. 
in  thickness  seldom  yield  good  ribbons.  The  same  is  true  of  very  thick 
ones.  A  small  sable  brush  held  in  the  left  hand  is  useful  in  keeping  the  first 
few  sections  fiat,  in  brushing  away  shreds  of  paraffin,  and  in  supporting  the 
ribbon  after  it  has  become  too  long  to  lie  on  the  knife.  Under  favorable 
conditions  and  with  careful  support,  ribbons  one  or  two  feet  in  length,  or  indeed 
much  longer,  may  be  cut  readily.  With  the  ordinary  sliding  microtome,  it 
is  safer  to  remove  the  chains  to  an  adjacent  sheet  of  clean  paper  when 
they  reach  a  length  of  ten  or  twelve  inches,  care  being  taken  that  the 
last  cut  two  or  three  sections  remain  undisturbed  on  the  knife  to  guide 
the  succeeding  ones.  The  under  surface  and  edge  of  the  knife  must  be  kept 
free  from  particles  of  paraffin,  otherwise  breaks  and  fissures  in  the  sections 
may  occur. 

As  the  cutting  proceeds,  it  may  become  necessary  to  retrim  the  block, 
since,  unless  the  sides  have  been  cut  strictly  vertical  and  not,  as  they  often 
are,  slightly  sloping,  the  border  of  parafilin  becomes  gradually  excessive. 
Then,  too,  the  front  and  back  edges  of  the  block  must  be  parallel,  otherwise 
the  ribbon  will  curve  instead  of  remaining  straight.  After  the  cutting  is  well 
under  way,  any  considerable  change  in  the  plane  of  section  should  be 
avoided  if  it  is  important  to  secure  a  complete  series,  as  such  change  usually 
necessitates  the  loss  of  several  sections.  If  the  embedding  mass  be  too  hard, 
the  sections  will  not  adhere  and,  of  course,  the  ribbon  not  form  ;  if  too  soft, 
the  sections  will  wrinkle  and  compress  and,  perhaps,  not  advance.  These 
defects  may  often  be  overcome  by  appropriate  changes  in  the  temperature  of 
the  room,  or  by  judicious  local  application  of  cold  or  heat  to  the  block  it- 
self. When  other  means  fail,  coating  the  block  with  paraffin  of  appropri- 
ate consistence  often  affords  relief.  Now  and  then,  especially  when  using 
rather  hard  paraffin  on  a  cold,  bright,  dry  day,  the  atmospheric  conditions 
are  such  that  sufficient  electricity  is  liberated  during  sectioning  to  disturb 
seriously  the  sections.  The  latter  may  break  apart  and  shorter  or  longer 
pieces  of  the  ribbon  attach  themselves  to  one  another  or  to  the  microtome 
or  knife.  A  repetition  of  this  exasperating  accident  may  usually  be  avoided 
by  charging  the  atmosphere  with  moisture,  often  conveniently  accomplished 
by  liberating  steam. 


APPENDIX.  399 


STAINING. 


The  object  of  staining  is  to  bring  out  the  various  structural  components 
of  the  tissues  by  taking  advantage  of  their  selective  affinities  for  certain 
dyes.  The  differentiation  usually  sought  has  as  its  first  object,  the  display 
of  the  nucleus;  next,  the  tingeing  of  the  cytoplasm;  and  thirdly,  the  exhi- 
bition of  the  intercellular  substances.  Of  the  large  number  of  staining 
methods  which  from  time  to  time  have  been  devised  to  meet  the  require- 
ments of  particular  lines  of  work,  only  a  few  of  the  most  reliable  and  gen- 
erally useful  will  be  here  given.  For  the  ordinary  needs  of  the  histologist, 
staining  with  hematoxylin,  followed  by  eosin,  leaves  little  to  be  desired, 
since,  when  successful,  the  nuclear  differentiation  is  sharp  while  the  cyto- 
plasm and  intercellular  substances  are  sufficiently  tinged  to  produce  clear 
and  instructive  pictures.  When,  as  in  the  case  of  embryos,  it  is  convenient 
to  stain  in  bulk  before  sectioning  in  paraffin,  borax-carmine  will  be  found  a 
most  reliable  dye,  possessing  penetration  and  uniformity  of  action  in  a  highly 
satisfactory  degree. 

Ehrlich's  Hematoxylin. 

Hematoxylin  crystals  (Griibler) 2  gm. 

Absolute  alcohol 100   cc. 

Distilled  water 100   cc. 

Ammonium  alum in  excess 

Glycerine 100    cc. 

Glacial  acetic  acid 10    cc. 

A.  Dissolve  the  hematoxylin  in  the  absolute  alcohol  and  let  stand  in 
loosely  corked  bottle  for  a  week,  where  the  sunlight  may  reach  it. 

B.  To  the  distilled  water  add  ammonium  alum  to  excess.  Add  A  to 
B,  while  stirring  vigorously.  After  several  days  add  the  glycerine  and  the 
glacial  acetic  acid  and  place  the  stain,  in  a  corked  bottle,  in  some  suitable 
position  insuring  exposure  to  direct  sunlight.  During  the  next  two  weeks 
uncork  for  several  days  at  a  time.  Direct  sunlight  is  important.  After  three 
months  the  stain  has  "  ripened  "  sufficiently  for  use.  If  kept  in  a  well-stop- 
pered bottle,  with  an  excess  of  alum,  the  reagent  retains  its  staining  powers 
for  years  and,  indeed,  improves  with  age.  Although  requiring  a  long  time 
until  ready  for  use,  Ehrlich's  hematoxylin  is  most  satisfactory,  staining 
well  after  all  the  usual  methods  of  fixation  and,  with  proper  precautions, 
being  permanent. 

This,  as  all  other  hematoxylin  stains,  should  be  filtered  immediately 
before  using.  Depending  upon  the  age  of  the  solution  and,  to  some  extent, 
upon  the  method  of  fixation,  sections  require  to  be  left  in  the  stain  from  5  to 
10  minutes.  They  are  then  washed  thoroughly  in  running  water  for  one  or 
two  hours.  If  deeply  colored  they  may  be  placed  in  a  dish  of  70  alcohol, 
acidulated  by  the  addidon  of  5  drops  of  hydrochloric  acid  to  each  100  cc.  of 
alcohol.  The  sections  remain  in  the  acid  bath,  in  which  they  turn  reddish, 
so  long  as  they  discharge  color,  keeping  them  moving  and  not  allowing 
them  to  adhere.  During  subsequent  washing  the  blue  color  is  restored, 
becoming  brighter  as  the  washing  progresses.  While  the  acid  alcohol  may 
be  omitted  with  sections  not  too  deeply  tinted,  full  staining  followed  by  the 
acid  alcohol  yields  sharp  nuclear  differentiation  and  brilliant  pictures.  The 
employment  of  the  acid  bath  may  be  recommended,  therefore,  as  a  routine 


400  APPENDIX. 

procedure.  Since  the  permanency  of  all  hematoxylin  preparations  depends 
tipon  the  elimination  of  every  trace  of  acid,  prolonged  and  thorough  washing 
is  essential,  otherwise  fading  will  occur.  If  after  short  washing  the  sections 
are  placed  for  a  few  minutes  in  water,  to  which  a  few  drops  of  ammonia  have 
been  added,  and  then  washed  thoroughly  in,  preferably  running,  water,  per- 
manency of  the  staining  is  assured. 

Bohmer's  Hematoxylin. 

Hematoxylin  crystals  (Griibler's) i  gm. 

Absolute  alcohol lo    cc. 

Ammonium  alum lo  gm. 

Distilled  water 200    cc. 

Dissolve  the  hematoxylin  in  the  absolute  alcohol  and  keep  in  stoppered 
bottle  for  24  hours.  Dissolve  the  alum  in  the  warm  distilled  water.  When 
cool,  gradually  add  the  dissolved  hematoxylin  while  stirring.  Place  in  an 
open  wide-mouth  bottle  or  dish,  protected  from  dust,  for  one  week,  when, 
after  filtering,  the  stain  is  ready  for  use.  Keep  in  a  tightly  corked  bottle 
and  filter  the  required  quantity  on  using.  Immersion  from  10  to  30  minutes 
usually  suffices  to  stain  to  the  required  degree.  This  solution  stains  well, 
although  not  uniformly  so  intensely  as  Ehrlich's,  and,  if  the  sections  are 
thoroughly  washed,  yields  permanent  results. 

Delafield's  Hematoxylin. 

Hematoxylin  crystals  (Griibler's) 4  gm. 

Absolute  alcohol 25    cc. 

Ammonium  alum 52  gm.  . 

Distilled  water 400    cc. 

Glycerine 100   cc. 

Methyl  alcohol 100    cc. 

The  hematoxylin  is  dissolved  in  the  absolute  alcohol  and  the  alum  in 
the  heated  distilled  water.  After  the  alum  solution  has  cooled,  the  hema- 
toxylin is  slowly  added  while  stirring  and  the  fluid  allowed  to  stand  in 
a  wide  open  vessel,  as  a  beaker  or  jar,  protected  from  dust  but  exposed 
to  light  and  air,  for  about  15  days.  Filter  and  add  glycerine  and  methyl 
alcohol.  Expose  to  the  light  until  the  stain  acquires  a  dark  purplish  tint, 
then  filter  and  keep  tightly  stoppered.  The  advantages  of  Delafield's 
hematoxylin  are  less  for  coloring  sections  than  for  staining  tissue  in 
bulk.  For  mass-staining,  the  freshly  filtered  solution  is  diluted  with  five 
to  ten  volumes  of  distilled  water,  in  which  the  pieces  of  tissue  remain 
from  24  to  48  hours,  or  longer,  until  uniformly  darkly  tinted  throughout. 
The  stained  tissue  is  rinsed  in  distilled  water  and  placed  in  acid  70  alcohol 
(5  drops  hydrochloric  acid  to  100  cc.  alcohol)  from  4  to  8  hours.  After 
this  differentiation  the  tissue  must  be  thoroughly  washed  in  running  water 
for  1 2  to  24  hours  to  remove  every  trace  of  acid  and  to  bring  out  the  rich 
blue  color. 

Contrast  staining  after  hematoxylin,  especially  when  the  latter 
has  been  limited  chiefly  to  the  nuclei,  adds  much  to  the  effectiveness  of  the 
preparation.  Such  double  staining  may  be  accomplished  by  treating  the 
hematoxylin  sections  with  solutions  of  acid  fuchsin,  Congo  red,  or  eosin.  The 
latter  answers  well  and  is   convenient.      The   most  satisfactory  results  are 


APPENDIX.  401 

obtained  with  "water  soluble"  eosin  (not  "alcohol  soluble"),  of  which 
a  .5  per  cent,  solution  in  70  alcohol  is  preferable,  the  sections  remaining 
until  decidedly  rosy,  usually  a  matter  of  1-2  minutes.  They  are  then 
transferred  to  70  alcohol,  in  which  they  remain  so  long  as  clouds  of  eosin 
are  discharged;  after  one  or  two  changes  of  70  alcohol  and  no  further 
color  is  given  off,  the  sections  are  passed  as  rapidly  as  thorough  dehy- 
dration will  permit  through  95  alcohol,  then  cleared  in  carbol-xylol,  and 
mounted  in  balsam,  as  presently  described. 

Grenacher's  Borax-Carmine. 

Carmine  (No.  40) 3  gm. 

Borax 4  gm. 

Distilled  water 100    cc. 

70  alcohol 100    cc. 

The  carmine  and  borax  are  rubbed  together  in  a  mortar  in  the  warmed 
distilled  water  until  dissolved.  After  cooling,  the  alcohol  is  added  and  the 
unfiltered  solution  placed  in  a  stoppered  bottle.  The  stain  is  not  ready  for 
use  until  it  has  stood  for  several  weeks.  The  quantity  required  for  staining, 
which  may  be  repeatedly  em^ ployed,  should  be  decanted  and  filtered  before 
using.  Although  sections  may  be  stained  with  the  solution,  its  chief  value 
is  for  mass-staining,  since  its  powers  of  penetration  and  uniform  coloration 
are  excellent.  Assuming  that  the  solution  is  to  be  put  to  this  use,  the 
object  is  transferred  from  70  alcohol  to  the  undiluted  stain,  in  which  it 
remains  24-48  hours  and  is  then  directly  placed,  that  is  without  washing, 
into  acid  alcohol  (8  drops  of  hydrochloric  acid  to  100  cc.  of  70  alcohol) 
for  8-24  hours,  or  longer,  depending  upon  the  size  and  density  of  the 
object.  The  purpose  of  the  acid  bath  is  to  differentiate  the  nuclei  and  fix 
the  stain.  Deep  staining  and  thorough  differentiation  produce  the  most 
satisfactory  preparations.  After  two  changes  of  70  alcohol,  the  object 
remaining  from  2-3  hours  in  each,  it  is  dehydrated  in  95  and  absolute 
alcohol  preparatory  to  embedding. 

Contrast  staining  after  carmine  may  be  carried  out  with  .5-1  per 
cent,  alcoholic  solutions  of  such  aniline  dyes  as  methylene  blue,  methyl 
violet,  or  methyl  green.  The  labor  and  time  involved  in  staining  serial  sec- 
tions on  the  slide,  as  of  course  must  be  done,  as  well  as  the  limited  endurance 
of  the  aniline  tint,  ordinarily  deter  from  double  staining.  Although  the 
second  dye  yields  pleasing  preparations,  unless  there  is  some  special  reason 
to  warrant  the  additional  labor,  the  contrast  color  may  be  omitted  with  serial 
sections  of  tissues  stained  in  bulk. 

Staining  sections  on  the  slide  is  necessary  when  it  is  desirable  to 
tinge  uncolored  paraffin  sections,  or  to  add  a  contrast  tint  to  those  stained 
en  masse  before  cutting.  Since  the  paraffin  must  be  removed  from  the  sec- 
tion before  the  stain  can  act,  it  is  evident  that  the  support  aft'orded  by  the 
embedding  mass  must  be  replaced  by  that  of  the  slide  before  the  paraffin  may 
be  removed  with  safety.  The  sections  must,  therefore,  be  fixed  to  the  slide, 
so  that  subsequent  manipulations  will  not  disturb  even  the  most  delicate  and 
isolated  parts  of  the  sectioned  object. 

All  slides  and  cover-glasses  used  for  mounting  preparations  must  be 
thoroughly  cleaned.  This  is  readily  accomplished  by  soaking  the  slides  and 
cover-glasses  in  strong  sulphuric  acid  for  5-10  minutes,  care  being  taken  to 
immerse  the  pieces  separately  and  not  to  allow  them  to  adhere.  They  are 
transferred,  piece  by  piece,  to  a  large  vessel  containing  water  and  washed 
26 


402  APPENDIX. 

thoroughly  under  the  tap,  with  occasional  separation  and  agitation  until  all 
traces  of  the  acid  are  removed.  The  slides  and  covers  are  then  placed  in  95 
alcohol  and  carefully  dried  by  wiping  with  a  clean  linen  cloth  of  appropriate 
thickness,  from  which  all  sizing  and  starch  have  been  removed  by  previous 
laundering.  Neglect  to  provide  properly  cleaned  slides  and  covers  mars 
many  valuable  preparations. 

The  method  of  fixing  to  the  slide  should  be  made,  if  possible,  to  serve 
an  additional  and  important  purpose,  namely,  to  expand  and  to  fiatten  out 
the  paraffin  sections,  which  very  often  are  slightly  compressed  and  wrinkled. 
To  mount  them  in  this  condition  may  seriously  interfere  with  their  later  use, 
as  in  the  case  of  serial  sections  of  embryos  in  making  reconstructions.  When 
the  fixed  sections  are  not  to  be  treated  with  watery  solutions,  a  convenient 
and  satisfactory  means  is  the  g-um  method,  carried  out  as  follows:  Of  a  satu- 
rated aqueous  solution  of  best  gum  arable  (a  crystal  of  thymol  being  added 
to  prevent  the  growth  of  fungi)  about  12  drops  are  added  to  30  cc.  of  dis- 
tilled water  and  thoroughly  shaken.  The  clean  slide  is  flooded  with  the 
solution,  care  being  taken  that  the  solution  does  not  run  over  the  edges,  and 
the  sections  are  floated  on  the  liquid,  all  parts  of  the  sections  being  separated 
from  the  slide  by  a  stratum  of  the  solution.  When  all  the  sections  are 
arranged,  the  slide  is  placed  on  a  warm  metal  plate  and  very  cautiously 
heated,  the  temperature  never  being  allowed  to  rise  to  the  melting  point  of 
the  paraffin,  the  object  being  to  secure  the  expansion  of  the  sections  while 
swimming  on  the  gum  solution.  They  expand  in  a  few  minutes.  The  excess 
of  fluid  is  then  drained  off,  the  sections  are  finally  rearranged  with  a  needle, 
and  the  slide,  protected  from  dust,  is  set  aside  to  dry.  The  latter  must  be 
thorough,  and  requires,  therefore,  some  hours,  a  good  plan  being  to  leave 
the  sections  undisturbed  over  night. 

The  albiimen-glycerine  mixture  is  another  excellent  fixative,  which  has 
the  advantage  over  the  gum  solution  of  being  unaffected  by  water.  This 
consists  of  equal  parts  of  the  strained  white  of  a  fresh  egg  and  glycerine, 
thoroughly  stirred  with  a  glass  rod.  A  small  drop  of  the  fixative  is  placed 
on  the  slide  and  spread  out  as  evenly  and  thinly  as  possible.  A  number  of 
slides  may  be  prepared,  dried,  and  suitably  stored  for  subsequent  use.  The 
albumen-coated  slide  is  now  covered  with  a  thin  stratum  of  water,  the 
sections  arranged,  and  the  slide  then  placed  on  the  top  of  the  oven,  or 
other  appropriate  warm  location,  where  the  sections  expand  and  the  water 
evaporates. 

Removing  the  paraffin  is  the  next  step  preparatory  to  staining  sections 
on  the  slide,  whatever  method  may  have  been  employed  to  secure  their  at- 
tachment. To  accomplish  this  the  slides  with  the  securely  affixed  sections 
are  immersed  for  a  few  minutes  in  xylol,  which  promptly  dissolves  the  par- 
affin and  leaves  the  cleared  sections  in  place,  freed  from  the  embedding  ma- 
terial. The  slides  are  then  transferred  to  95  alcohol  to  displace  the  xylol, 
after  a  few  minutes  passed  to  fresh  alcohol,  and  then  carried  through  70  per 
cent,  spirit  into  the  staining  fluid,  if  that  be  an  alcoholic  solution,  or  into 
water,  in  case  the  stain  be  chiefly  aqueous.  The  most  convenient  receptacles 
for  exposing  a  number  of  section-loaded  slides  to  the  various  solutions  are 
the  square  glass  "  staining-jars,"  provided  with  grooves  and  covers,  which 
allow  several  pairs  of  slides,  placed  back  to  back,  to  be  stood  on  end  and  im- 
mersed at  one  time.  The  disadvantages  of  the  square  jars  in  common  use 
are  the  danger  of  damaging  the  sections  by  contact  with  the  grooves  and  the 
lack  of  a  ground-joint  cover  to  prevent  evaporation,  as  when  containing  xylol 
or  absolute  alcohol.      Although  less  readily  procured,  the  small  cylindrical 


APPENDIX.  +03 

preparation  jars,  with  ground  stoppers,  meet  every  requirement.  When  of 
the  proper  height  and  diameter,  they  permit  three  pairs  of  shdes  to  be  im- 
mersed at  one  time  with  convenience  and  safety  and  enable  volatile  fluids  to 
he  used  repeatedly  without  deterioration.  After  the  staining  has  been  com- 
pleted according  to  the  details  of  the  method  selected,  the  sections  are  passed 
through  jars  containing  95  and  absolute  alcohol  to  insure  complete  dehydra- 
tion prior  to  clearing  and  mounting. 

Clearing  the  sections  is  necessary  to  render  the  otherwise  more  or 
less  opaque  tissue  transparent  and  suitable  for  microscopical  examination. 
If  the  tissue  contains  no  trace  of  water,  it  may  be  mounted  in  pure  Canada 
balsam,  the  usual  medium  in  which  objects  are  preser\ed  as  permanent  prepa- 
rations, directly  from  fresh  absolute  alcohol,  the  latter  being  graduallv 
replaced  by  the  highly  refracting  balsam  and  the  required  transparency 
thereby  secured.  If,  however,  the  alcohol  be  of  insufficient  strength,  as 
often  occurs  after  exposure,  it  mixes  with  the  balsam  imperfectly  and 
turbidity  results.  In  order  to  obviate  such  accident,  it  is  more  con- 
\'enient  to  employ  some  clearing  agent  of  suitable  refraction  as  a 
go-between,  which  will  mix  with  tlie  alcohol,  on  the  one  side,  and  with  the 
balsam  on  the  other. 

The  means  employed  to  secure  this  transparency  depends  upon  the 
condition  of  the  tissue,  whether  infiltrated  with  the  embedding  mass,  as  in 
the  case  of  celloidin  sections,  or  free  from  such  support,  as  in  the  case  of 
jiaraffin  sections  attached  to  the  slide. 

Celloidin  sections  are  best  cleared  in  a  carbol-xvlol  mixture,  consisting 
of  one  part  of  pure  carbolic  acid  to  three  parts  of  xylol.  The  dehydrated 
sections  are  transferred  to  this  fluid  from  the  strong  alcohol  and  after 
a  few  minutes  become  transparent.  If,  however,  the  required  transparencv 
fails  to  appear  after  fi\-e  or  ten  minutes,  it  may  be  concluded  that  the 
dehydration  has  been  insufficient  to  allow  the  penetration  of  the  clearing- 
solution.  The  sections  must  be  returned,  therefore,  to  the  alcohol.  In  all 
cases  thorough  dehydration  must  be  insured  by  adequate  treatment  with 
strong  alcohol. 

Paraffin  sections,  after  the  removal  of  the  paraffin  by  immersion  in 
xylol,  are  transferred  to  some  clearing  fluid,  oil  of  turpentine  answering  well 
and  being  inexpensixe. 


MOUNTING   AND   FINISHING. 

Unless  required  for  only  cursory  examination,  the  sections  are  mounted 
in  some  medium  suitable  for  further  study  and  preservation.  The  most  sat- 
isfactory medium  for  general  use  is  pure  Canada  balsam,  which  is  supplied 
in  convenient  collapsible  metal  tubes,  from  which  the  required  amount  may 
be  pressed.  In  the  case  of  tissues  cut  in  celloidin,  the  individual  sections 
are  transferred  from  the  dish  of  clearing  solution  (carbol-xylol)  to  a  clean 
slide  by  means  of  a  thin  metal  section-lifter,  a  clean  mounted  needle  being- 
used  in  guiding  and  holding  the  section  on  the  lifter,  as  well  as  in  transfer- 
ring it  on  the  slide  from  the  lifter.  When  the  latter  is  broad,  the  blade 
should  be  perforated  to  facilitate  raising  the  section  from  the  fluid,  as  well  as 
draining  off  the  clearing  solution.  After  being  placed  on  the  slide,  the 
superfluous  carbol-xvlol  having  been  carefully  removed  by  filter  paper,  the 
section  is  finally  arranged  bv  judicious  use  of  the  needle  and  a  drop  of  balsam 
gently  pressed  out  from  the  tube  upon  the  middle  of  the  preparation. 


404  APPENDIX. 

If  the  balsam  has  the  proper  consistence,  about  that  of  syrup,  the  shde  is 
held  for  a  few  moments  over  a  spirit  flame,  the  slight  heating  facilitating  the 
distribution  of  the  balsam  over  the  section.  The  cover-glass,  previously 
scrupulously  cleaned  and  in  readiness,  is  grasped  between  the  clean  blades 
of  the  forceps,  held  for  a  moment  over  the  spirit  flame,  and  gently  lowered 
on  the  slide  in  such  manner  that  the  left  edge  of  the  cover  is  brought  first  into 
contact  with  the  balsam.  As  soon  as  this  contact  is  made,  a  moment's  pause 
is  advantageous  to  allow  the  balsam  to  spread  out  beneath  the  cover,  which 
is  then  steadily  and  slowly  lowered  into  position  over  the  specimen  until  the 
entire  space  between  the  slide  and  cover  is  filled  with  the  mounting  medium. 
If,  after  very  gentle  pressure  on  the  cover-glass  with  a  clean  needle,  part  of 
this  space  remains  unfilled,  additional  balsam  must  be  run  in  until  the  space 
is  completely  occupied  by  the  mounting  medium.  Care  must  be  exer- 
cised lest  any  excess  of  balsam  get  on  the  surface  of  the  cover  ;  otherwise 
it  does  not  interfere  with  the  immediate  examination  of  the  preparation. 
A  slight  edging  of  balsam  around  the  cover  is  useful,  since  it  dries 
much  sooner  than  the  medium  beneath  the  cover  and  thereby  adds 
materially  to  the  fixation  of  the  latter.  While  avoiding  as  far  as  possible 
the  imprisonment  of  air-bubbles,  should  these  be  present  after  the  cover  is 
in  place,  they  need  cause  no  concern,  as  they  usually  spontaneously 
disappear  during  the  next  twelve  hours,  unless  enclosed  within  some  recess 
or  fold  in  the  section. 

While  the  preparation  may  be  examined  under  the  microscope  as  soon 
as  properly  mounted,  care  being  taken  to  keep  it  horizontal  to  avoid  possible 
slipping  of  the  cover-glass,  no  attempt  should  be  made  to  finish  it  until  the 
balsam  around  the  edges  of  the  cover  has  sufficiently  hardened  to  preclude 
displacement.  Ordinarily  this  requires  some  weeks,  but  the  process  of  hard- 
ening may  be  facilitated  greatly  by  subjecting  the  freshly  mounted  specimen, 
in  some  suitable  place  protected  from  dust,  to  continuous  gentle  heat.  After 
a  few  days  of  such  drying,  although  the  balsam  in  the  centre  of  the  prepara- 
tion may  remain  fluid,  the  cover  will  be  so  securely  fixed  that  the  specimen 
may  be  finished  and  labelled.  If  the  superfluous  balsam  be  considerable,  as 
much  as  possible  should  be  removed  by  the  knife,  care  being  taken  not  to 
touch  the  cover-glass,  and  the  slide  finally  cleaned  with  a  cloth  moistened 
with  xylol.  Disturbing  the  cover-glass  will  likely  ruin  the  preparation  and, 
hence,  this  danger  is  to  be  constantly  borne  in  mind. 

After  the  slide  has  been  cleaned,  not  forgetting  the  under  side,  labels 
are  attached  at  the  ends.  Data  of  importance,  regarding  tissue,  source, 
method  of  fixation,  staining,  and  date,  should  be  noted  on  one  label  and  never 
be  entrusted  to  memory;  failure  to  observe  such  precautions  is  often  a  con- 
stant source  of  uncertainty  and  regret  in  later  years.  When  preparations 
include  serial  sections,  numbers  or  letters  on  the  labels  should  indicate  the 
sequence  of  the  slides.  Much  time  may  be  saved  by  marking  the  slides  as 
to  order  with  a  writing  diamond  before  they  receive  the  sections.  The 
second  label  is  convenient  for  memoranda  concerning  special  features  shown 
by  the  preparation.  Where  many  specimens  accumulate,  some  system  of 
card-catalogue  well  repays  in  convenience  for  future  reference.  The  various 
methods  of  storing  microscopical  preparations  are  matters  of  individual 
preference  and  expediency,  provided  two  essentials  are  observed — that  the 
slides  lie  horizontal  and  are  protected  from  light.  With  proper  precautions 
against  the  presence  of  acid  in  the  sections,  even  hematoxylin  preparations 
may  be  preserved  for  many  years  without  deterioration. 


APPENDIX.  405 


SYNOPSIS    OF   STANDARD   METHODS. 

Assuming  that  the  tissue  is  of  moderate  density,  has  been  fixed  in  Zenker's  fluid 
and  preserved  in  70  alcohol,  with  due  precautions  for  the  removal  of  the  deposits  of 
mercury  (page  390),  and  that  the  size  of  the  piece  to  be  cut  is  approximately  a  surface 
of  I  sq.  cm.  by  .5  cm.  thick,  the  steps  in  the  two  standard  methods — the  celloidin  and 
the  parafhn — are  as  follows: 

Celloidin.  I  Paraffin. 

1.  95  alcohol,  24  hours.  '      i.  Stain   tissue  en  masse  in   borax-car- 

2.  Absolute  alcohol,  24  hours.  mine,  24-48  hours. 

3.  Mixture  absolute  alcohol  and  ether,         2.  Acid  alcohol,  24-4S  hours.  Delafield's 

24  hours.  hematoxxlin    may   be  used    (page 

4.  Thin  (C)  celloidin,  1-2  days.  400  )  instead  of  borax-carmine. 

5.  Medium  (B)  celloidin,  i-2da\s.  ■     3.  70  alcohol,  24  hours. 

6.  Thick  (A)  celloidin,  2-4  days.  1     4.  95  alcohol,  24  hours. 

7.  Mount  on  fibre-block.  5.  Absolute  alcohol,  24  hours. 

8.  Harden  in  No  alcohol,  24  hours.  6.  Chloroform,  3-6  hours. 

9.  Cut  sections,  wet  with  80  alcohol.  1     7.  Saturated     solution     of    parafhn    in 

10.  Stain  in  Ehrlich's  hematoxylin,  5-10   I  chloroform,  6-12  hours. 

minutes.  \     8.  Melted  paraffin  in  oven  at  52°  C,  2 

11.  Wash  in  water,  5-10  minutes.  1  hours. 

12.  Acidulated    70    alcohol,    about  one-   '     9.   Fresh  melted  paraffin,  4-6  hours. 

half  minute;  wash  in  water.  10.  Embed  in  fresh  paraflin. 

13.  Ammoniated  water,  until  again  blue.        11.  Cut  sections,  dry. 

14.  Wash  thoroughly  in  water.  12.  Expand  sections  on  slide. 

15.  Stain  in  eosin,  1-2  minutes.  ,    13.  Dry    slides    to    attach    sections,     12 

16.  70  alcohol,  until  no  color  is  discharged.  hours. 

17.  95  alcohol,  5  minutes.  14.  Xylol  to  remove  paraffin,  3  minutes. 

1 8.  Clear  in  carbol-xylol.  i    15.  Turpentine  spirits,  3  minutes. 

19.  Mount  in  balsam.  i    16.  Mount  in  balsam. 

If  paraffin  sections  are  to  be  stained  on  the  slide,  steps  1-3  are  omitted,  the  tissue 
embedded  and  cut  and  the  sections  attached  to  the  slides.  The  paraffin  is  then  re- 
moved and  the  tissue  prepared  for  the  stain  by  placing  the  slides  in: 

a.  xylol,  3  minutes.  e.  Stain  as  desired. 

b.  95  alcohol,  3  minutes.  f.  Dehydrate  in  ascending  alcohols, 

c.  80  alcohol,  3  minutes.  g.  Clear  in  turpentine. 

d.  70  alcohol,  3  minutes.  h.  Mount  in  balsam. 


SOME   USEFUL   SPECIAL    METHODS. 

\A^eigert's  Stain  for  Medullated  Nerve-Fibres. — This  method  is 
especially  adapted  to  display  ner\'e-fibres  invested  with  medullary  substance, 
the  latter  appearing  dark  or  slate-blue,  while  the  gray  matter  and  nerve-cells 
are  stained  a  light  yellowish  tint.  The  nervous  tissue,  in  pieces  not  too 
large  and  as  fresh  as  possible,  is  suspended  in  a  relati\'ely  large  quantity  of 
Miiller's  fluid  (page  391)  for  4-6  weeks,  changed  daily  during  the  first  week 
and  occasionally  thereafter.  The  pieces  of  spinal  cord  or  brain  are  then 
transferred  directly,  without  washing  in  water,  into  70  alcohol,  followed  by 
90,  and  kept  in  the  dark  for  almost  a  week,  the  alcohol  being  renewed  sev- 
eral times  and  always  when  turbid. 


4o6  APPENDIX. 

The  Weigert  method  requires  the  employment  of  three  solutions,  the 
mordant  (A),  the  stain  (B),  and  the  differentiation  fluid  (C),  made  respec- 
ti\elv  as  follows: 

A.  Saturated  aqueous  solution  of  neutral  cupric  acetate^ 

approximately  lo  gm.  of  the  copper  salt  to  loo  cc.  dis- 
tilled water. 

B.  Hematoxylin  crystals  (Griibler) i  gm. 

Absolute  alcohol lo  cc. 

Distilled  water loo  cc. 

The  hematoxylin  is  dissolved  in  the  alcohol,  added  to  the  dis- 
tilled water  and  boiled  ;  after  cooling,  filter. 

C.  Borax 2  gm. 

Potassium  ferric  cyanide 2.5  gm. 

Distilled  water 100  cc. 

After  being  cut  in  celloidin,  the  sections  are  placed  for  12  hours  in  the 
copper-solution,  composed  of  equal  parts  of  freshly  filtered  A  and  distilled 
water.  They  are  then  transferred  directly  to  the  stain,  B,  in  which  they  re- 
main for  12—36  hours,  followed  by  differentiation  in  C.  The  over-stained 
sections  remain  in  this  fluid  until  color  is  no  longer  extracted  and  the  con- 
trast between  the  white  and  gray  matter  is  well  accentuated,  ordinarily  from 
30-60  minutes  sufihcing.  The  sections  are  then  thoroughly  washed  in  run- 
ning water  for  8-12  hours  and  carried  through  ascending  alcohols  until  de- 
hydrated, when  they  are  cleared  in  carbol-xylol  and  mounted  in  balsam.  If 
the  staining  has  been  unsuccessful,  the  same  sections  may  be  placed  in  Miil- 
ler's  fluid  for  24  hours,  rinsed  in  distilled  water,  and  treated  with  the  copper 
and  subsequent  solutions  as  before. 

Sodium  Carminate. — Satisfactory  as  is  hematoxyhn-eosin  for  usual 
purposes,  this  stain  is  inadequate  for  really  good  demonstrations  of  the  nerve- 
cells  and  nerve-fibres  of  the  brain  and  spinal  cord.  Excellent  preparations 
of  these  elements  may  be  made  by  the  following  method  :  Small  pieces  of 
the  fresh  tissue,  not  over  i  cm.  thick,  are  fixed  in  an  excess  of  Miiller's  fluid, 
changed  daily  during  the  first  week  and  occasionally  afterwards.  After  four 
weeks  the  tissue  is  transferred  directly,  without  washing,  into  a  sufficient 
quantity  (40-50  cc. )  of  i  p.c.  aqueous  solution  of  sodium  carminate,  in 
which  it  remains  3  days  with  frequent  shakings.  The  pieces  of  stained  tissue 
are  washed  for  24  hours  in  running  water,  and  then  dehydrated  in  ascending- 
alcohols,  embedded  in  celloidin  and  cut.  When  successful,  the  cells  and  the 
axis-cylinders  appear  red  and  sharply  differentiated  on  a  light  ground,  the 
cells  of  Purkinje  in  such  preparations  being  beautifully  shown.  Lack  of  pene- 
tration and  over-staining  at  the  surface  are  the  most  common  sources  of 
failure. 

Chrome-Silver  Impregnation. —  The  introduction  by  Golgi  of 
methods  of  silver  impregnation  has  added  an  important  means  of  demon- 
strating the  astonishing  richness  and  extent  of  the  ramifications  of  the  neu- 
rones and  of  the  neuroglia-cells.  Of  the  several  procedures  suggested  by 
Golgi  the  so-called  rapid  method  is  here  given.  It  is  most  successful  when 
applied  to  the  nervous  tissues  of  the  foetal  or  new-born  animal,  and,  at  best, 
is  capricious  and  uncertain. 

The  fresh  young  tissue  is  fi.xed  either  in  bichromate-formalin  (4  parts  of 
3.5  p.c.  aqueous  solution  of  potassium  bichromate  to  i  of  40  p.c.  formalin), 
or  in  Golgi  s  fluid  (9  parts  of  3.5  p.c.  aqueous  solution  of  potassium  bichro- 
mate to  I  part  of  2  p.c.  aqueous  solution  of  osmic  acid). 


APPENDIX.  407 

Depending  upon  the  object  in  view  and  the  age  of  the  animal,  the  tissue 
remains  in  the  fixing  fluid  from  2-15  days.  If  foetal  or  very  young,  2-3 
days  suffice  for  the  neuroglia-cells,  3-5  days  for  the  nerve-cells,  and  5-7 
days  for  the  collaterals.  Older  tissues  require  longer  immersion,  adult  brain 
being  left  in  the  fluid  from  8-15  clays.  After  rinsing  for  a  few  moments 
with  distilled  water  and  quickly  drying  ofi  with  filter  paper,  the  pieces,  not 
over  .5  cm.  thick,  are  placed  in  i  p.  c.  aqueous  solution  of  silver  nitrate.  In 
this  they  remain  2-3  days  or  longer,  during  which  a  dark  precipitate  ap- 
pears. In  order  to  determine  the  probable  success  of  the  impregnation,  free- 
hand sections  are  cut  and  examined  in  95  alcohol  under  the  microscope.  If 
satisfactory,  the  tissue  is  dehydrated  in  absolute  alcohol,  embedded  in  cel- 
loidin  as  rapidly  as  possible,  and  cut  in  95  alcohol  into  sections  not  too  thin. 
The  sections  are  passed  to  absolute  alcohol  for  thorough  dehydration,  then 
to  creosote  for  10  minutes,  and,  after  remaining  5  minutes  longer  in  xylol, 
finally  placed  on  the  slide,  dried  rapidly  with  filter  paper,  and  covered  with 
a  large  amount  of  balsam.  The  slide  is  now  heated  carefully  over  a  flame 
until  the  balsam  will  set  as  soon  as  cool.  Before  this  occurs,  however,  a 
heated  cover-glass  is  applied  and  the  section  permanently  mounted.  This 
manipulation,  suggested  by  Huber,  insures  the  removal  of  all  moisture  and 
the  probable  permanency  of  the  preparation,  thus  avoiding  the  unsatisfactory 
plan  of  keeping  the  specimens  covered  with  only  balsam  and  unprotected  by 
a  cover-glass. 

Should  the  preliminary  examination  disclose  insufficient  silver  deposit, 
as  is  frequently  the  case,  the  block  of  tissue  is  returned  to  the  bichromate- 
osmic  solution  for  2-3  days  and  again  subjected  to  the  silver  solution.  If 
necessary,  this  procedure  may  be  repeated  a  number  of  times  with  the  same 
tissue  until,  perchance,  the  results  are  satisfactory.  This  may  be  regarded  to 
be  the  case,  if  the  desired  nervous  elements  appear  as  sharply  defined  dark 
figures  on  a  light,  almost  colorless  background.  Even  in  otherwise  success- 
ful preparations,  many  parts  of  the  section  may  be  almost  useless  owing  to 
disturbing  deposits  of  silver-precipitate.  Notwithstanding  its  uncertainty, 
the  Golgi  method  yields  such  remarkable  demonstrations  of  the  ner\'ous  ele- 
ments, that  really  successful  results  amply  repay  perseverance. 

Gold  Staining. — The  gold-chloride  method  is  useful  to  demonstrate 
nerve-endings,  such  as  the  motor  plates  in  striated  muscle.  Small  pieces  of 
fresh  tissue,  not  over  5  mm.  in  dimension,  are  treated  with  the  gold  solution 
prepared  as  follows:  4  parts  of  i  per  cent,  aqueous  solution  of  gold-chloride 
and  I  part  of  formic  acid  are  heated  to  boiling  three  times.  The  tissue  is 
placed  in  this  solution  when  cool  for  i  hour,  in  the  dark.  It  is  then  rinsed 
in  distilled  water  for  half  a  minute  and  transferred  to  diluted  formic  acid  ( i 
part  acid  to  4  parts  distilled  water)  and  allowed  to  stand  in  the  light,  but 
not  direct  sunlight,  18-48  hours.  By  the  end  of  this  period  the  exterior  of 
the  tissue  has  acquired  a  dark  violet  color.  Small  fragments  may  be  teased 
in  glycerine  and  if  successfully  stained  may  be  mounted  permanently  in  the 
same  ;  or,  if  desirable,  the  entire  piece  may  be  hardened  in  ascending  alcohols, 
sectioned  and  mounted  in  balsam.  The  use  of  steel  instruments  during  these 
manipulations  must  be  avoided,  thin  glass  rods  of  suitable  size  being  sub- 
stituted. Admirable  preparations,  showing  nerves  and  connective  tissue  cells, 
may  be  made  from  the  corneae  of  small  animals,  the  tissue  being  separated 
into  thin  lamellae  before  mounting. 

Silver  Staining. — Staining  with  argentic  nitrate,  as  contrasted  with 
Golgi  impregnations,  is  employed  especially  to  differentiate  the  cell-boun- 
daries of  endothelium,  since  by  its  employment  the  intercellular  cement  sub- 


4o8  APPENDIX. 

stance  is  demonstrated  as  dark  lines.  The  method  is  of  further  use  in  bring- 
ing to  view  the  lymph-spaces  within  the  dense  connective  tissues,  as  the  cor- 
nea, in  which  the  spaces  then  appear  as  irregular  light  figures  surrounded 
by  the  brown  ground-substance. 

When  the  mesothelium,  covering  for  instance  the  mesentery,  is  to  be 
displayed,  the  tissue  is  cut  from  the  recently  killed  animal  and  carefully 
transferred  by  glass  rods  to  i  per  cent,  aqueous  solution  of  argentic  nitrate. 
After  immersion  for  5-10  minutes,  according  to  the  thickness  of  the  object, 
the  tissue  is  rinsed  in  distilled  water,  placed  in  a  porcelain  dish  containing 
distilled  water,  and  stood  in  the  direct  sunlight  until  reduction  of  the  silver 
is  completed.  This  is  indicated  by  a  decided  reddish-brown  tint  and  usually 
requires  8-15  minutes.  The  tissue  is  then  transferred  to  a  small  dish  of  dis- 
tilled water  to  which  a  few  granules  of  sodium  chloride  have  been  added,  the 
purpose  of  the  latter  being  to  arrest  the  action  of  the  silver.  After  10  min- 
utes, the  stained  tissue  is  placed  for  6-10  hours  in  70  alcohol,  in  the  dark, 
followed  by  dehydration  in  alcohol,  clearing  and  mounting  in  balsam.  After 
the  reduction  of  the  silver,  staining  with  hematoxyhn  adds  to  the  interest  of  the 
preparation  by  bringing  out  the  nuclei,  which  otherwise  are  only  faintly  seen. 

When  the  endothelium  of  the  blood-vessels  is  to  be  stained,  the  method 
recommended  by  Huber  may  be  followed  with  advantage.  After  the  escape 
of  the  blood  following  incision  of  the  exposed  heart  of  an  anesthetized  animal, 
a  glass  or  hard  rubber  canula  is  inserted  into  the  thoracic  aorta  and  the  ves- 
sels injected  with  a  i  per  cent,  solution  of  argentic  nitrate.  After  fifteen  min- 
utes, the  inferior  vena  cava  is  cut  immediately  below  the  heart  and  a  4  per 
cent,  solution  of  formalin  (10  parts  commercial  formalin  to  90  parts  of  dis- 
tilled water)  is  injected  into  the  aorta  through  the  same  canula.  The  injec- 
tion of  the  formalin  washes  out  the  superfluous  silver  solution,  thereby 
avoiding  disturbing  precipitates,  and  fixes  the  vessels  while  distended-.  The 
desired  tissue  is  then  cut  from  the  animal,  care  being  taken  to  remove  the 
structures  to  be  examined  in  pieces  sufficiently  supported  to  prevent  undue 
distortion,  immersed  in  4  per  cent,  formalin  and  exposed  to  direct  sunhght. 
While  the  latter  is  not  necessary,  reduction  of  the  silver  taking  place  slowly 
in  diffuse  daylight,  the  rapid  reduction  effected  by  sunlight  is  favorable  to 
sharp  and  well  differentiated  histological  pictures  ;  it  should,  therefore,  be 
employed  whenever  possible.  After  dehydration,  small  flat  pieces  of  the 
tissue  are  cleared  in  carbol-xylol  and  mounted  in  balsam.  If  protected  from 
strong  daylight,  such  preparations  may  be  preser\'ed  for  years  with  little 
deterioration. 

Injecting  Blood- Vessels. — In  order  to  demonstrate  the  distribution 
of  the  smaller  blood-vessels  and  the  capillaries,  advantage  is  taken  of  some 
means  to  fill  the  blood-channels  with  a  colored  substance.  The  injection- 
mass  must  meet  two  requirements— be  transparent  and  not  diffuse  through 
the  walls  of  the  smallest  vessels.  Successful,  that  is  complete,  injection  of 
the  capillaries  requires  considerable  experience  and,  even  at  best,  is  attended 
with  an  element  of  uncertainty,  since  the  condition  of  the  tissues,  particularly 
of  the  vessels,  influences  the  freedom  with  which  the  injecting  fluid  runs. 

Two  injecting  masses  are  commonly  employed,  carmiiie-gelatin  and 
Berlin  blue.  When  successful  the  former  yields  very  beautiful  preparations, 
but  has  the  disadvantage  of  requiring  to  be  used  while  hot  and  with  heated 
tissue,  in  order  to  prevent  untimely  solidification.  The  beginner  will  find 
Berlin  blue  more  convenient,  since  it  is  used  cold,  runs  well  in  unwarmed 
tissues,  and  does  not  extravasate.  The  results,  moreover,  are  equally  in- 
structive, although  perhaps  less  striking.     The  injection  fluid  is  readily  pre- 


APPENDIX.  409 

pared  by  dissolving-  3  gm.  soluble  Berlin  blue  (  (iriibler)  in  600  cc.  distilled 
water.  A  smoothly  working  syringe  of  200-300  cc.  capacity,  with  tightly 
fitting  stop-cock  and  several  appropriate  canulae,  is  the  best  instrument,  since 
the  educated  hand  of  the  operator  is  the  surest  gauge  of  the  pressure  that 
may  be  applied  with  safety. 

A  small  animal,  such  as  a  young  rabbit  or  kitten,  is  chloroformed  and 
then  bled  to  death  by  opening  the  heart  or  some  large  vein,  so  that  the  \es- 
sels  are  emptied  as  far  as  possible.  It  is  advisable  to  undertake  at  first  the 
injection  of  a  single  organ,  as  the  liver,  kidney,  or  lung,  rather  than  of  the 
entire  animal.  The  organ  is  not  removed  and  is  disturbed  only  as  much  as 
may  be  necessary  to  expose  sufificiently  its  chief  artery.  Into  this  a  canula 
of  appropriate  size  and  fitted  with  a  stop-cock  is  inserted  through  a  small 
slit  and  securely  tied.  In  order  to  avoid  the  introduction  of  air,  the  canula 
should  be  filled  with  the  injecting  fluid,  and  the  stop-cock  turned,  before 
I)eing  introduced  into  the  vessel.  When  the  canula  is  in  place  and  securetl, 
the  syringe  is  filled,  fitted  to  the  canula,  the  stop-cock  opened,  and  the  fluid 
gently  forced  into  the  vessels.  Great  care  must  be  exercised  lest  sudden 
and  excessive  pressure  rupture  the  delicate  vessels  and  the  fluid  escape.  If 
all  goes  well,  the  injected  organ  soon  begins  to  assume  a  bluish  tint,  but 
until  the  tissue  appears  deeply  and  uniformly  colored  the  capillary  injection 
is  incomplete.  Before  removing  the  canula,  the  large  vessels  should  be 
secured  with  ligatures.  The  organ  is  then  removed  from  the  animal  and 
placed  in  Miiller's  fluid  or  70  per  cent,  alcohol  for  some  days  before  cutting 
into  pieces.  Sections  of  injected  organs  must  not  be  too  thin,  but  sufficiently 
thick  to  include  complete  capillary  loops  or  networks.  In  the  case  of  the 
lungs,  after  injecting  the  blood-vessels,  the  tissue  should  be  moderately  dis- 
tended by  forcing  the  fixing  fluid  through  the  air-tubes,  which  are  then 
ligated. 


It  may  be  repeated,  that  the  purpose  of  these  pages  concerning  micro- 
scopical technique  is  to  present  a  few  methods  which  are  satisfactory  and 
thoroughly  trustworthy  for  the  majority  of  histological  examinations.  The 
student  is  urged  to  perse\'ere  with  those  here  given  until  he  has  repeatedly 
carried  the  manipulations  to  the  successful  results  which  they  are  capable  of 
yielding. 


INDEX. 


Accessory  spleens,   ii6 

Acer\'iilus  cerebri,  308 

Adamantoblasts,   140 

Adipose   tissue,  jq 

Alimentary  canal,   129 

Anieloblasts,  140 

Amitosis,    10 

Aniphiaster,   S 

Anabolism,  5 

Anaphases  of  cell-division,  9 

Aponeuroses,  63 

Appendage,   vesicular  of   female.  253 

Appendix  epididymidis,  235 

testis,   235 
Arachnoid,   314 

Pacchionian  bodies,  314 
Arbor  vita-  cerebelli,  294 
Areolar  tissue,  2"] 
Arrectores  piloruni.  ^2'6 
Artery,  lielicine  of  penis,  240 
Arteries,  of  large  size,  90 

structure  of,  8g 
Articulations,  51 
Atrio-ventricular   Inmdle,   102 
Auditory  ossicles,  367 
Auerbach's   plexus,    163 
Axis-cylinder,  of  nerves,  68 

processes.  65 
Baillarger's  stripe,  302 
Bartiiolin's  glands,  262 
Basement  membrane,   121: 
Basket-cells   of   cerebellum,   295 
Bertin's    colunms    of   kidney,    206 
Betz's  cortical  cells,  304 
Bladder,  217 
Blastodermic  layers,   13 

derivatives  of,   15 

vesicle,  12 
Blood.  94 

clot,  98 

colored  cells,  94 

colorless  cells,  96 

fibrin,  98 

granules.  98 

serimi,  98 
Blood-crystals,  98 
Blood-plaques,  97 
Blood-plates,  97 
Blood-vascular  sj'stem,  87 
Blood-vessels,  development  of,  99 

endothelium    of.    88 

muscle  of,  88 

nerves   of,  89 

nutrient  vessels  of,  89 

structure  of,  87 
Bodies.   Nissl's,  66 


Bohmer's  hematoxylin,  400 
Bone,  34 

absorption  of,  46 

blood-vessels  of,  42 

cancellated,   35 

central  spongy,  46 

compact,  35 

development  of.  42 

endochondral  development,  43 

growth   of,   50 

intracartilaginous,  43 

intramembranous  development,  49 

membrane,  43 

perforating  fibres.  2J 

subperiosteal,  47 
Bone-marrow,  39 

primary,   45 

red,  40 

yellow,    42 
Bowman's  glands,  382 

membrane,  340 
Brain,   279 

blood-vessels  of,  307 
•  choroid  plexuses,  313 

general   description   of,   279 
Brain-sand.  308 
Brain-stem,  279 
Bronchi,   192 
Bruch's  membrane,  343 
Brunner's  glands,  166 
Buccal  glands,  147 
Bulbo-urethral   glands,   244 
Bulbus  vestibuli,  262 
Bursse,  64 

Capillaries,   structure   of.   92 
Cardiac  muscle.  55 

nerve-endings.  86 
Carotid  body  or  gland,   117 
Cartilage,  articular,  51 

cells  of.  31 

elastic,    Z2> 

fibrous.  33 

growth  of,  2>~ 

hyaline,  31 

matrix  of.  31 
Cells,  connective  tissue,  23 

ependynial,   71 

fat,   30 

free  connective  tissue,  24 

general   considerations  of,   I 

mast.  25 

origin  of,   1 1 

of  Paneth,  168 

pigment,  24 

plasma,  25 

structure   of,   2 

411 


412 


INDEX. 


Cell-division,  6 

amitotic  or  direct,  lo 

mitotic  or  indirect,  7 
Cell-plate,  9 

Central   nervous  system,  266 
Centriole,  s 
Centroplasm,  5 
Centrosome,  4 
Centrosphere,  5 
Cerebellum,  294 

basket-cells,  295 

cortex,  294 

cortical  nerve-fibres,  297 

internal  nuclei,  298 

neuroglia,  208 

Purkinje  cells,  294 
Cerebral  crura,  291 

peduncles,  291 
crusta,    292 
substantia  nigra,  292 
tegmentum,  292 
Cerebrospinal  fluid,  311 
Cerebrum,  299 

basal  ganglia,  304 

caudate  nucleus,  305 

cortical  gray  matter,  299 

cortical  nerve-cells,  300 

cortical  nerve-fibres,  303 

cortical  variations,  304 

giant   pyramidal   cells,   304 

internal  nuclei,  304 

lenticular  nucleus,  305 

optic  tbalamus,  306 

pineal   body,   308 

Rolandic  cortex,  304 

stratum  zonale,  301 
Ceruminous  glands,  364 
Chondroclasts,   45 
Choroid  plexuses  of  brain,  313 
Chromafiine  cells,  iiS 
Chromosomes,  8 

reduction  of,  9 
Chromatin,  4 
Clasmatocytes,  25 
Claudius'  cells  of  ear,  ^yp 
Clitoris,   262 

Cloquet's  canal  in  eye,  356 
Coccygeal  body  or  gland,   118 
Cohnheim's  fields,  59 
Collagen,  25 
Colostrum,  264 
Conarium,   308 
Conjunctiva,  357 
Connecting  fibres,  9 
Connective  tissue,  21 
Contour  lines  of  Owen,  133 
Cornea,  339 
Corneal  corpuscles,  340 
Corpus  dentatus  of  cereljellum,  2c 

lateum,  249 

striatum,  305 

trapezoides,   291 
Corpuscles,  genital,  81 

Golgi-Mazzoni,  83 

Grandrv's,  80 

Herbsf's,   83 


Corpuscles,    Meissner's,   80 

Pacinian,    82 

tactile,  80 
Corti's  membrane  of  ear.  378 

organ  of  ear,  376 
Cowper's  glands,  244 
Crusta  petrosa,   135 
Crystalline  lens,  354 
Cuticle  of  skin,  317 
Cylindrical  end-bulbs,  82 
Cytoplasm,  structure  of.  2 
Deciduse  menstrualis,  259 
Deiter's  cells  of  ear,  378 
Delafield's  hematoxylin,  400 
Demours'  membrane,  340 
Dental  papilla,   137 
Dentine,  133 

Descemet's  membrane,  340 
Deutoplasm,    1 1 
Diploe,  35 
Diplosome,  5 
Duct,  bile,  ejaculatory,  237 

Gartner's,   253 

paraurethral,  261 

spermatic,  235 
Ductuli  aberrantes,  235 
Dura  mater,  312 
Ear,  362 

auditory  cells,  '377 

auditory  ossicles,   367 

auricle,  362 

basilar  membrane,  27^ 

blood-vessels  of,  379 

bony  cochlea,  370 

bony  semicircular  canals, '370 

canalis  reuniens,  374 

cerumen,  364 

ceruminous  glands,  364 

cochlear  duct,  373 

cristas   acttsticse,   373 

endolymph,   369 

Eustachian  tube,  367 

external,  362 

external  auditory  canal,  363 

internal,  369 

ligamentum  spirale,  375 

maculse  acusticse,   372 

mastoid  cells,  369 

membrana  tectoria,  378 

membranous  labyrinth,  371 

middle,  365 

nerves  of,  379 

organ  of   Corti,  376 

osseous  labyrinth,  369 

otolitli  membrane,  372 

perilymph,  369 

pillars  of  Corti,  377 

Reissner's    membrane,    374 

scala  tympani,  374 

scala  vestibuli,  374 

spaces  of  Nuel,  378 

tubal  tonsil,  368 

tympanic  cavity,  365 

tympanic  membrane,  365 

vestibule,  369 
Ear-stones,  372 


INDEX. 


413 


Ectoderm,   13 

Ehrlich's  hematoxylin,  399 

Ejaculatory  duct,  237 

Elastin,  2"] 

Eleidin,  320 

Elementary  tissues,   i6 

Embryonic  area,  13 

Enamel,  131 

Enamel-organ,  139 

Encephalon,   279 

End-bulbs,  81 

Endocardium,  loi 

Endochondral  development  of  bone,  43 

Endolymph  of  ear,  369 

Endometrium,  256 

Endomysium,  59 

Endoneurium,  ']2 

Endoplasm,  3 

Endoskeleton,  34 

Endothelium,  20 

End-plate  of  motor  nerves,  85 

Entoderm,    13 

Eosinophilcs,  97 

Eosin-bodies  of  cerebellum,  297 

Ependyma,   71 

Epicardium,  103 

Epidermis,  317 

Epididymis,  22,}t 

canal  of,  233 

efferent   ducts,   233 
Epimysium,   59 
Epineurium,  72 
epiphysis,  308 

ossification  of,  47 
Epiploic  appendages,  172 
Epithelial  bodies,   199 
Epithelium,  16 

ciliated,    ig 

columnar,  18 

germinal,  245 

glandular,  19 

modified,   18 

pigmented,   19 

rod,  20 

squamous,  16 
Eponychium,  332 
Epoophoron,  252 
Erythroblasts,    99 
Erythrocytes,   development   of,   99 

nucleated,  41 

size  of,  95 

structure  of,  94 
Eustachian  tube,  367 
Eye,  zzi  _ 

anterior  chamber,  357 

aqueous  humor,  357 

canal  of  Schlemm,  340 

choriocapillaris.  343 

choroid,  342 

ciliary  body,  343 

ciliary  muscle,  344 

ciliary  processes,  344 

ciliary  ring,  344 

conjunctiva,  357 

cornea,  339 

crystalline  lens,  354 


Eye,  fibrous  tunic,  338 

fovea  centralis,  347 

hyaloid   canal,   356 

hyaloid  membrane,  355 

iris,  345 

lachrymal  passages,  361 

lachrymal   sac,   'sdi 

lamina  cribrosa.  353 

lamina  fusca,  338 

ligament  of  lens,  356 

macula  lutea,  349 

Miiller's  fibres,  350 

muscles  of  iris,  345 

nervous  tunic,  346 

optic  nerve,  353 

ora  serrata,  352 

pectinate  ligament,  340 

posterior  chamber,  357 

retina,  346 

rods  and  cones,  348 

sclera,   338 

spaces  of  Fontana,  341 

tear-gland,  361 

vascular  tunic,  341 

vense  vorticosse,  343 

vitreous  body,  355 

yellow  spot,  347 
Eyelids,  357 

cilia,  357 

tarsal  glands,  359 

tarsal  plates,  358 
Exoplasm,  3 
Exoskeleton,   34 
Fallopian  tube,  254 
Fasciae,  63 
Fasciculus  antero-lateralis  superfic,  277 

cerebello-spinalis,  z'"] 

cerebro-spinalis,  278 

cuneatus,  268 

gracilis,  268 

posterior  longitudinalis.  288 

solitarius,  287 
Fat-cells,  30 

serous,  31 
Fat-organs,   31 
Fibres,  connective  tissue,  25 

elastic,  26 

white,  25 
Fibrin,  98 
Fibrocartilage,  2,21 
Fibrous   tissue,  22, 
Fillet,  median,  decussation,  284 
Fontana's  spaces,  341 
Formatio   reticularis,   of   medulla.   285 
Fovea  centralis  of  eye,  347 
Flernming's  solution,  391 
Frontal  sinus,  384 
Funiculi,   of  nerves,  "^2 
Gall-bladder.   182 
Ganglia,   development  of,  78 

spinal,  74 

sympathetic,  74 
Ganglion-crest,  78 
Gartner's  duct,  253 
Gay's  glands,  336 
Genital  corpuscles,  81 


414 


INDEX. 


Germ-cells,  ii 

Germ-centres  of  lymph-nodes,   ic 

Germ-layers,   13 

Germinal  epithelium,  245 

Giraldes'  organ,  235 

Glands,   122 

of  Bartholin,  262 

of  Bowman,  382 

of  Brminer,  166 
•        buccal,  147 

bulbo-urethral,  244 

bulbo-vestibular,  262 

cardiac  of  stomach,   160 

ceruminous,    364 

ciliary,  336 

circumanal,  336 

compound  alveolar,  127 

compound  tubular,  124 

cutaneous,  332 

development  of,  127 

epithelium    of,    125 

gastric,   158 

of  Gay,  336 

labial,   147 

lachrymal,  361 

of  Lieberkuhn,  168 

lingual,   145 

of  Littre,  220 

mammary,  262 

Meibomian,  359 

molar,  147 

of  Moll,  359 

mucous,  126 

nerves  of.  127 

of  Nuhn,   14s 

oesophageal,  155 

olfactory,  382 

oral,   146 

palatal,   151 

parotid,   148 

sebaceous,  3;^^ 

serous,  125 

simple  alveolar,  127 

simple  tubular,   124 

sublingual,  149 

submaxillary,   149 

solitary  of  intestine,  168 

sweat,  334 

tubo-alveolar,  124 

vessels  of,  127 

of  Zeiss,  359 
Glomeruli  of  cerebellum,  297 

of  kidney,  209 
Glomus  caroticum,   117 

coccygeum,   118 
Golgi-Mazzoni  corpuscles,  8? 
Grandry's  corpuscles,  80 
Grenachcr's  borax-carmine,  401 
Growth,  s 
Gustatory  cells,  387 
Hairs,  323 

development  of,  328 

erector   muscle   of,   328 

follicle,  325 

growth  of,  329 

root-sheaths,  326 


Hairs,  structure  of,  324 
Hassall's  corpuscles,  201 
Haversian  canals,  38 

space,    49 

system,  35 
Heart,  loi 

blood-vessels  of,  103 

development  of,  104 

endocardium,  loi 

epicardium,  103 

myocardium,  102 

nerves  of.  103 

valves  of,  102 
Hematoconia,  98 
Hematoidin  crystals,  98 
Hemin  crystals,  98 
Hemolymph  nodes,  112 
Hensen's  cells  of  ear.  378 
Herbst's  corpuscles,  83 
Henle's  layer,  hair-follicle,  326 

sheath,  of  nerves,  72 
His's  bundle  of  heart,  102 
Howship's  lacunae,  46 
Huxley's  layer,  hair-follicle,  326 
Hyaloplasm,  3 
Hydatid  of  Morgagni,  253 
Hypophysis  cerebri,  309 
Ileo-colic  valve,  173 
Incremental   lines  of   Salter,   133 
Interglobular  spaces  of  tooth,  135 
Intervertebral  disk,  51 
Intramembranous   ]:)one-development,   49 
Involuntary  muscle,  53 
Irritability,  6 

Islands  of  Langerhans,  185  ' . 
Jacobson's  organ,  384 
Joints,  structure  of,  51 
Karyokinesis,  7 
Karyoplasm,  3 
Karyosomes,  4 
Katabolism,  5 
Keratohyalin,  319 
Kidney,  205 

architecture  of,  205 

blood-vessels  of,  213 

columns  of  Bertin,  206 

details  of  tubules,  209 

glomeruli,  209 

lymphatics  of,  215 

nerves  of,  215 

pelvis  of,  215 

substance  of.  207 

supporting  tissue  of,  213 

uriniferous  tubules,  207 
Kupffer's  cells  in  liver,  181 
Labial  glands,  147 
Labra  glenoidalia,  52 
Lachrymal  apparatus,  360 

gland,    361 

passages,  361 

sac,  362 
Lamina  fusca  of  eye,  338 
Langerhans'   islands,   185 
Large  intestine,  171 

epiploic  appendages,  172 
.  ileo-colic  valve,    173 


INDEX. 


415 


Large   intestine,   vcrmif.   appendix,    173 
Larynx,    187 

cartilages  of,  187 

vocal  cords,   188 
Lemniscus  medialis,  284 
Leucocytes,  97 

development  of,  100 
Ligamentum  circulare  dentis,   136 
Lingual  glands,  145 

tonsil,  145 
Linin,  4 

Lip.   structure   of,    129 
Lipochromes,  24 
Liquor  folliculi,  247 
Littre's   glands   of  urethra,   220 
Liver,   176 

bile-capillaries,  179 

biliary  passages,   181 

blood-vessels  of,  177 

cells  of.  178 
Lungs,  191 

air-spaces,   194 

architecture  of,  191 

blood-vessels  of,  195 

bronchioles,   192 

lobule,   191 

nerves  of^  196 

respiratory  epithelium,  194 

unit   of,   192 
Luschka's  gland,  118 
Lymph-nodes,    109 
Lymph-nodules,  108 
Lymph-spaces,    106 

perivascular,  107 
Lymph-vessels,  107 
Lymphatic  system,  105 

development  of,   112 
Lymphatics,  afferent,  109 

efferent,  109 
Lymphocytes,  97 
Lymphoid  tissue,  108 

germ-centres  of.  108 
lymph-nodes.  log 
lymph-nodules.  loS 
ALacula  lutea  of  eye,  349 
Alannnary  glands,  262 

milk,  264 
Alartinotti's  cells,  303 
Mast-cells,  25 
Mastoid  cells,  369 
Maxillary  sinus,  384 
Medulla  oblongata.  282 

arcuate   fibres.   284 
decussation,  motor,  283 
decussation,  sensory,  284 
formatio  reticularis,  285 
pyramidal  decussation,  283 
posterior  nuclei.  283 
Medullary  sheath  of  nerves,  68 
Megakaryocytes,  41 
Meibomian  glands.  359 
Meissner's  corpuscles,  80 

plexus,   163 
Membrana  propria,  121 

tyinpani,  365 
Membrane-bones,  43 


Membrane,  fenestrated,  27 
of  Nasmyth,  132 
synovial,  52 
Meninges,  311 
Mesenciiyma.  21 
Mesoderm,  13 
Mesodcrmic  somites.  14 
Mesothelium,  21 
Metabolism,  5 

Metaphase  of  cell-division.  8 
Metaplasm,  3 
Microscopical  technique.  389 

celloidin,  embedding.  393 
chrome-silver  impregnation,  40;) 
cutting  sections,  396 
fixation  of  tissues,  389 
fixing  sections,  402 
gold  staining,  407 
injecting  blood-vessels,  408 
mounting  and  finishing.  403 
paraffin,  embedding,  394 
preservation  of  tissues,  389 
serial  sections,  397 
silver  staining,  407 
staining,   399 
staining  on  slide,  401 
Weigert's  method,  405 
Microsomes,  3 
Mid-body,  9 
Milk,  264 
Mitosis,  7 

synopsis  of.   10 
Mitotic  figure,  8 
Molar  glands.   147 
Moll's  glands,  359 
Morgagni's  hydatid,  253 
Motor  nerve-endings.  85 
Mouth,  glands  of,   146 
Mucous  membranes,  119 
epithelium,   120 
muscularis,  121 
nerves  of,   121 
submucous  tissue,  121 
tunica  propria,  120 
vessels  of,    121 
Mucous  tissue,  22 
Miiller's  ciliary  muscle,  344 
fibres  of  retina,  350 
fluid,  391 
Muscles,  attachment  of,  62 
cardiac,  55 

contractile    fibrillcT.   58 
development  of,  61 
fasciculi.  59 
fibre-cells,  54 
fields  of  Cohnheim,  59 
general  considerations,  53 
nonstriated,  53 
red,  59 
striated,  58 

structure  of  striated,  58 
Aluscle-columns,  58 
Muscle-nuclei.   59 
Muscle-spindles.   83 
Muscularis  mucosas,  121 
Myeloblasts,    100 


4i6 


INDEX. 


Myelocytes,  41 
Myelospongium,  7(> 
Myocardium,  102 
Myo-fibrils,  55 
Myometrium,  257 
Nails,  330 

growth  of,  331 
Nasmyth's  membrane,   132 
Nerve-cells,  66 

Nerve-endings,  encapsulated,  80 
free  sensory,  79 
motor,  85 
sensory,  79 
Nerve-fibres,  68 
afferent,  64 

efferent,   64 
Nerve-terminations,  79 
Nerve-trunks,  71 
Nerves,  development  of,  TJ 
Nervous  system,  essential  parts,  64 
Nervous  tissue.  64 

development  of,  76 
Neurilemma,  68 
Neuroblasts,  T] 
Neuroepithelium,   20 
Neuroglia,  70 

Neuromuscular   endings,  83 
Neurone,  motor,  64 

sensory,  64 

structure  of,  65 

types  of,  67 
Neurotendinous  endings,  85 
Nissl's  bodies,  66 

Nonstriated  muscle,  nerve-endings, 
Normoblasts,    99 
Nose,  accessory  air-spaces,  384 

cavernous  tissue  of,  383 

external,  380 

Jacobson's  organ,  384 

mucous  membrane  of,  380 

olfactory   glands,   382 

olfactory  region,  381 

respiratory  region,  382 
Nuclear  spindle,  8 
Nuclein,  4 
Nucleolus,  4 
Nucleus  of  abducens  nerve,  290 

ambiguus,  287 

caudatus,  305 

cuneatus,  284 

dentatus  of  cerebellum,  298 

dorsal  vago-accessory,  286 

emboliformis,  298 

of  facial  nerve,  291 

fastigii,  298 

globosus,   298 

gracilis,  283 

of  hypoglossal  nerve,  286 

lateralis,  287 

lenticularis,  305 

of  oculomotor  nerve,  291 

pontile,  290 

ruber,  293 

structure  of,  3 

of  trigeminus  nerve,  291 
Nuel's  spaces  of  ear,  378 


Nulin's   glands,    145 
Odontoblasts,  136 
CEsophagus,  154 

glands  of,  155 
Oliva,  285 
Olivary  nuclei,  accessory,  285 

inferior,  285 
Oolemma,  11 
Optic  thalamus,  306 
Oral  cavity,  mucosa  of,  129 
Organs  of  internal  secretion,  226 
of  respiration,  187 
of  special  sense,  315 
Orth's  fluid,  391 
Osseo-mucoid,   35 
Osteoblasts,  41,  45 
Otoconia,  ■^J}, 
Otoliths,  372 
Ovary,  245 

corpus  luteum,  249 
Graafian  follicles,  245 
medulla  of,  250 
Oviduct,  254 
Ovula  Nabothi,  257 
Ovum,  human,  248 
parts  of,   II 
primary,  246 
Owen's  contour  lines,   133 
Pacchionian  bodies,  314 
Pacinian  corpuscles,  82 
Palate,  151 

glands  of,  151 
Pancreas,  182 
duct,  184 

interalveolar  cell-areas,  185 
islands  of  Langerhans,  185 
Paneth's   cells,    168 
Panniculus  adiposus,  317 
Papillae,  circumvallate,  143 
filiform,  143 
foliate,   144 
fungiform,   143 
of  tongue,  143 
Paradidymis,  235 
Paraganglia,  75 
Paraganglion  caroticum,  118 
Paralinin,  4 

Parathyroid    bodies,    199 
Paraurethral  ducts,  261 
Pareleidin,  320 
Paroophoron,  253 

inferior  cerebellar,  286 
Peduncle,  middle  cerebellar,  288 

superior  cerebellar,  293 
Penis,  239 
Pericardium,  104 
Pericementum,   135 
Perichondrium,  32 
Perilymph  of  ear,  369 
Perimetrium,  258 
Perimysium,  59 
Perineuriimi,   72 
Periosteum,  38 

osteogenetic  layer,  47 
Peritoneum,  175 
Peyer's  patches,  169 


INDEX. 


417 


Pliarynx.    152 
Pia  niatcr,  312 
Pigment   cells,  24 
Pineal  hody,  308 

eye,  309 
Pituitary  body,  309 

anterior   lobe,   ,'oy 
posterior  lobe,  310 
Plasma-cells,  25 
Plasmosome,  4 
Pleura,   ig6 
Plexus  of  Auerbacli,  163 

choroid  of  brain,  313 

of  Meissner,  163 

myospermaticus,  238 
Polar  field,  8 
Pons  Varolii,  288 

tegmental  part.  290 
ventral  part.  288 
Prophases  of  cell-division,  8 
Prostate  gland,  241 

secretion  of,  243 
Prostatic  calculi,  242 
Protoplasm,   2 
Pnrkinje  fibres  of  heart,   102 

nerve-cells,  294 
Pyramidal   tracts,   decussation,   28^ 
Pyrenin,  4 

Ranvier's  nodes,  of  nerves,  69 
Red  muscles,  59 
Red  nucleus,  293 
Reissner's  membrane,  374 
Remak's   nerve-fibres.   70 
Renal   duct.  215 
Reproduction.  5 
Reproductive  organs,   female,  245 

male,  227 
Restiform  body,  286 
Rete  Malpighi,  318 
Reticular  tissue,  22 
Retina,  346 
Retzius'  stripes,  132 
Riolan's  muscle,  358 
Roof-nuclei  of  cerebellum,  298 
Rudimentary  organs  of  female,  252 
Ruffini's  nerve-endings,  85 
Ruysch's  membrane,  343 
Salivary  corpuscles,   150 

glands,  146 
Salter's  lines.   133 
Sarcolemma,   57 
Sarcostyles,  58 
Sarcous  substance.  57 
Schlemm's    canal    in    eyeball,    340 
Schmidt-Lantermann   segments,  69 
Schreger's  lines  in  dentine,  133 

stripes  in  enamel,   132 
Schwann's  sheath,  68 
Scleral  corpuscles,  338 
Sebaceous   glands,    t,t,7, 
Segmentation.    12 
Seminal  vesicle.  237 
Sertoli's  cells  of  testicle,  230 
Sharpey's  fibres  of  bone.  37 
Sheath  of  Henle,  72 
Sinusoids,   93 


Skene's  tubes,  261 
Skin,  315 

corium,  316 
cutaneous  ridges.  316 
epidermis,  317 
glands  of.  332 
nerves  of,  322 
pigmentation  of,  320 
stratum   germinativum,  318 
tela  subcutanea.  317 
vessels  of.  321 
Small   intestine,   163 

aggregated  nodules,   169 
Brunner's  glands,  ifi6 
glands,    166 
lactcals,   165 

Lieberkiihn's   glands,    168 
lymph-nodules,    168 
Peyer's  patches,   169 
plicae  circulares.   166 
solitary  nodules,   168 
valvulse  conniventes,  166 
villi,   164 
Somatopleura,   14 
Somites,  mesodermic,  14 
Spermatic  duct,  235 
Spermatoblasts,  231 
Spermatocytes.  231 
Spermatogenesis,  230 
Spermatogones.  230 
Spermatozoa,  232 
Sperm-crystals.  244 
Sphenoidal  sinus,  384 
Spinal  cord.  266 

1)lood-vessels,   279 
Burdach's  tract,  277 
central   canal.  273 
Clarke's  colunm.  271 
direct  cerebellar  tract,  277 
fibre-tracts,   274 
Goll's  tract,  277 
Gowers'  tract,  277 
gray   matter.   268 
ground-bundle.    278 
membranes,    311 
nerve-cells.    2O9 
neuroglia.  272 
pyramidal  tracts.  278 
spino-thalamic  tract,  277 
substantia  gelatinosa.  273 
white  matter.   273 
Spireme  or  skein.  8 
Splanchnopleura,  14 
Splanchnoskeleton,  34 
Spleen,  113 

accessory.   116 
lobules  of,  114 
pulp  of,  115 
Spongioblasts,    77 
Spongioplasm.  3 
StilHng's  canal  of  eye,  356 
.Stomach,  157 

cardiac  glands,  160 
fundus  glands.  158 
pyloric  glands,  159 
pyloric  sphincter,  161 


418 


INDEX. 


Stomata    of    serous    membranes,    21 
Stratum   Malpighi,  318 
Striated  muscle,  nerve-endings,  85 
Stripes  of  Retzius,  132 

of  Schreger,  132 
Subarachnoid  space,  311 
Subdural  space,  311 
Sublingual  gland,  149 
Submaxillary  gland,  149 
Substantia  gelatinosa  of  medulla,  2^ 

of  spinal  cord,  273 
Substantia  nigra.  292 
Suprarenal  body,  222 

chromaffin  cells,  224 
cortex  of,  223 
medulla  of,  224 
Suprarenals,  accessory,  225 
Sweat-glands,  334 
Sylvian  aqueduct,  291 
Synovial  membranes,  52 

villi,  52 
Tactile  'cells,  80 

corpuscles,  80 
Tapetum    cellulosum,    343 

fibrosum,  343 
Taste-buds,  385 

nerves  of,  387 
Teeth,    131 

alveolar  periosteum,  135 

cementum,  135 

dentinal  tubules,  134 

dentine,    133 

development  of,  137 

enamel,  131 

interglobular  spaces,  135 

nerves  of,  136 

odontoblasts,   136 

pericementum,  135 

peridental  membrane,  135 

pulp,   136 

vessels  of.  136 

tooth-sac,  141 
Tellyesnizcky's  fluid,  390 
Telophases  of  cell-division,  9 
Tendon,  28 

cells,  28 

sheaths,  63 

spindles,  85 
Testicle,  227 

appendages  of,  235 

seminiferous  tubules,  229 

spermatogenesis,  230 
Thymus,  200 

atrophy   of,   203 

Hassall's  corpuscles,  201 

thymic  bodies,  201 
Thyroid  body,  197 
Tigroid  substance,  66 
Tissue,  adipose,  29 

areolar,  27 

connective,  21 

dense  fibrous,  27 

elementary,    16 

epithelial,   16 


Tissue,  iibrous,  23 

mucous,  22 

muscular,  53 

nervous,    64 

reticular,  22 
Tomes's  granule  layer,  135 

processes,  140 
Tongue,   142 

glands  of.  145 

muscles  of.  142 

papillae  of.  143 
Tonsils,    faucial,    153 

lingual,   14s 

palatine,    153 

pharyngeal,  154 

tubal,   154 
Tooth-development,  cementum,  141 

dentine,   137 

enamel,  140 

permanent    teeth,    141 
Trachea,   189 

glands  of,  191 

muscle  of,   191 
Trophoblast,   12 
Tuberculum  Rolandi,  284 
Tunica  propria,   120 
Ureter,  215 
Ureteral  sheath,  217 
Urethra,  219 

female,  221 

glands  of,  220 

male,  219 
Uterus,  255 

cervical  glands,  256 

changes  during  pregnancy,  259 

glands  of,  256 

menstrual  changes,  258 
Uveal  tract  of  eyeball,  341 
Vagina,  259 
Valves,  of  veins,  92 
Vas   deferens,  235 

ampulla    of,    235 
Vasa  vasorum,  89 
Vater-Pacinian  corpuscles,  82 
Vesicular  appendages,  253 
Veins,  structure  of,  91 

valves  of,  92 
Vermiform   appendage,   173 
Vital  phenomena,  5 
Vitellus,  II 
Volkmann's  canals,  38 
Vascular  system,  87 
Vulva,  260 

labia  majora,  260 

labia  minora,  261 

tubes  of  Skene,  261 

vestibule,  261 
Wharton's   jelly,    22 
Witch-milk,  264 
Zeiss'  glands,  359 
Zinn's  arterial  circle,  329,  339 

zonula  of  lens,  354 
Zona  pellucida,  247 

radiata,  248 


/^6/ 
/9/a 


4** 


